Plotting graphs for structural equation models

tidySEM offers a user-friendly, tidy workflow for plotting graphs for SEM models. The workflow is largely programmatic, meaning that graphs are created mostly automatically from the output of an analysis. At the same time, all elements of the graph are stored as data.frames, which allows swift and easy customization of graphics, and artistic freedom. Some existing graphing packages automatically create a layout, but are very difficult to customize. Particularly for complex SEM models, it may be preferable to make the layout by hand, including only nodes one wishes to plot, and reporting the rest in a comprehensive table of coefficients (e.g., one obtained through table_results().

The tidySEM workflow

The workflow underlying graphing in tidySEM is as follows:

Run an analysis, e.g., using lavaan::sem() or MplusAutomation::mplusModeler(), passing the output to an object, e.g., fit
Plot the graph using the function graph(fit), or customize the graph further by following the optional steps below.
Optionally Examine what nodes and edges can be extracted from the fit model object, by running get_nodes(fit) and get_edges(fit)
Optionally Specify a layout for the graph using get_layout()
Optionally, prepare graph data before plotting, by running prepare_graph(fit, layout). Store the resulting graph data in an object, e.g., graph_data
Optionally, access the nodes and edges in graph_data using nodes(graph_data) and edges(graph_data)
Optionally, modify the nodes and edges in graph_data using nodes(graph_data) <- ...and edges(graph_data) <- ...

This workflow ensures a high degree of transparency and customizability. Objects returned by all functions are “tidy” data, i.e., tabular data.frames, and can be modified using the familiar suite of functions in the ‘tidyverse’.

Example: Graphing a CFA

Step 1: Run an analysis

As an example, let’s make a graph for a classic lavaan tutorial example for CFA. First, we conduct the SEM analysis:

library(lavaan)
HS.model <- ' visual  =~ x1 + x2 + x3
              textual =~ x4 + x5 + x6
              speed   =~ x7 + x8 + x9 '
fit <- cfa(HS.model, data=HolzingerSwineford1939)

Step 2: Plot the graph

At this point, we could simply plot the graph:

graph_sem(model = fit)

This uses a default layout, provided by the igraph package. However, the node placement is not very aesthetically pleasing. One of the areas where tidySEM really excels is customization. Because every aspect of the graph is represented as tidy data (basically, a spreadsheet), it is easy to move nodes around and create a custom layout.

Optional step 3: Customizing the layout

In tidySEM, the layout is specified as a matrix (grid) with node names and empty spaces (NA or ""). There are essentially three ways to specify the layout:

Automatically, from the fit model
Manually in R
In a spreadsheet program

Automatically generate layout for a model

The example above used an automatically generated layout for the fit model (lavaan or Mplus). If you open the help file for the function by running ?graph_sem, you can see that the default argument is layout = get_layout(x = model), where model refers to the model argument of the same function.

The get_layout() function automatically generates a layout for a fit model object. To get this layout as a matrix, you can run:

get_layout(fit)
#>      [,1] [,2]      [,3] [,4] [,5]    [,6]     [,7] [,8]
#> [1,] NA   NA        NA   NA   NA      "visual" NA   NA  
#> [2,] NA   "textual" NA   NA   "speed" "x1"     "x2" "x3"
#> [3,] "x4" "x5"      "x6" "x7" "x8"    "x9"     NA   NA  
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

The get_layout() function relies on igraph::layout_as_tree() to place the nodes. Other layout functions from the igraph package can be used by specifying the layout_algorithm = ... argument:

get_layout(fit, layout_algorithm = "layout_in_circle")
#>      [,1] [,2] [,3]      [,4]    [,5]
#> [1,] NA   NA   "x5"      NA      NA  
#> [2,] NA   "x6" "x4"      "x3"    NA  
#> [3,] NA   "x7" "textual" "x1"    "x2"
#> [4,] "x8" "x9" "visual"  "speed" NA  
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"
get_layout(fit, layout_algorithm = "layout_on_grid")
#>      [,1]     [,2]      [,3]    [,4]
#> [1,] "x6"     "x7"      "x8"    "x9"
#> [2,] "x2"     "x3"      "x4"    "x5"
#> [3,] "visual" "textual" "speed" "x1"
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

Specifying layout manually in R

Manually specifying the layout can be done by providing node names and empty spaces (NA or ""), and the number of rows of the desired layout matrix. For example:

get_layout("c", NA,  "d",
           NA,  "e", NA, rows = 2)
#>      [,1] [,2] [,3]
#> [1,] "c"  NA   "d" 
#> [2,] NA   "e"  NA  
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

Of course, it is also possible to simply define a matrix using matrix().

Specifying layout in a spreadsheet program

Specifying the layout in a spreadsheet program is very user-friendly, because one can visually position the nodes, e.g.:

To obtain the layout matrix, one can save the spreadsheet as .csv file and load it in R like this:

read.csv("example.csv")

Alternatively, one can select the layout as in the image above, copy it to the clipboard, and then read it into R from the clipboard. This works differently on Windows and Mac.

On Windows, run:

read.table("clipboard", sep = "\t")

On Mac, run:

read.table(pipe("pbpaste"), sep="\t")

#>   V1     V2 V3
#> 1 x1     x2 x3
#> 2    visual

Examples of user-defined layout

We can specify a simple layout for two hypothetical nodes x and y is generated as follows:

get_layout("x", "y", rows = 1)
#>      [,1] [,2]
#> [1,] "x"  "y" 
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

For a mediation model, one might specify a layout like this:

get_layout("", "m", "",
           "x", "", "y", rows = 2)
#>      [,1] [,2] [,3]
#> [1,] ""   "m"  ""  
#> [2,] "x"  ""   "y" 
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

For a three-item CFA model, one might specify:

get_layout("", "F", "",
           "y1", "y2", "y3", rows = 2)
#>      [,1] [,2] [,3]
#> [1,] ""   "F"  ""  
#> [2,] "y1" "y2" "y3"
#> attr(,"class")
#> [1] "layout_matrix" "matrix"        "array"

And for the CFA model we estimated above:

lay <- get_layout("", "", "visual","","textual","","speed","", "",
                  "x1", "x2", "x3", "x4", "x5", "x6", "x7", "x8", "x9", rows = 2)

We could plot the CFA model with this custom layout as follows:

graph_sem(fit, layout = lay)

Step 4: Customize nodes and edges

For the simple model above, it is easy to verify the names of the nodes and edges from the syntax above: The nodes consist of three latent variables (visual, textual, and speed), and nine observed variables (x1-x9). The edges are nine factor loadings - and three latent variable correlations, included by default. We can confirm which nodes are available by running get_nodes():

get_nodes(fit)
#>       name shape   label
#> 1   visual  oval  visual
#> 2  textual  oval textual
#> 3    speed  oval   speed
#> 4       x1  rect      x1
#> 5       x2  rect      x2
#> 6       x3  rect      x3
#> 7       x4  rect      x4
#> 8       x5  rect      x5
#> 9       x6  rect      x6
#> 10      x7  rect      x7
#> 11      x8  rect      x8
#> 12      x9  rect      x9

And for the edges:

get_edges(fit)
#>       from      to arrow   label connect_from connect_to curvature
#> 1   visual      x1  last    1.00           NA         NA        NA
#> 2   visual      x2  last 0.55***           NA         NA        NA
#> 3   visual      x3  last 0.73***           NA         NA        NA
#> 4  textual      x4  last    1.00           NA         NA        NA
#> 5  textual      x5  last 1.11***           NA         NA        NA
#> 6  textual      x6  last 0.93***           NA         NA        NA
#> 7    speed      x7  last    1.00           NA         NA        NA
#> 8    speed      x8  last 1.18***           NA         NA        NA
#> 9    speed      x9  last 1.08***           NA         NA        NA
#> 10      x1      x1  both 0.55***           NA         NA        NA
#> 11      x2      x2  both 1.13***           NA         NA        NA
#> 12      x3      x3  both 0.84***           NA         NA        NA
#> 13      x4      x4  both 0.37***           NA         NA        NA
#> 14      x5      x5  both 0.45***           NA         NA        NA
#> 15      x6      x6  both 0.36***           NA         NA        NA
#> 16      x7      x7  both 0.80***           NA         NA        NA
#> 17      x8      x8  both 0.49***           NA         NA        NA
#> 18      x9      x9  both 0.57***           NA         NA        NA
#> 19  visual  visual  both 0.81***           NA         NA        NA
#> 20 textual textual  both 0.98***           NA         NA        NA
#> 21   speed   speed  both 0.38***           NA         NA        NA
#> 22  visual textual  none 0.41***           NA         NA        60
#> 23  visual   speed  none 0.26***           NA         NA        60
#> 24 textual   speed  none 0.17***           NA         NA        60

Customizing node and edge labels

The get_nodes() and get_edges() functions both call table_results() on the fit model object to get information about the nodes and edges. Both functions have a label argument which can either be a character string referencing a column of the output of table_results(), or an expression that is evaluated in the context of that output.

This allows you to customize node and edge labels. For example, maybe you want to combine the parameter estimate and its confidence interval into an edge label:

get_edges(fit, label = paste(est, confint))
#>       from      to arrow             label connect_from connect_to curvature
#> 1   visual      x1  last 1.00 [1.00, 1.00]           NA         NA        NA
#> 2   visual      x2  last 0.55 [0.36, 0.75]           NA         NA        NA
#> 3   visual      x3  last 0.73 [0.52, 0.94]           NA         NA        NA
#> 4  textual      x4  last 1.00 [1.00, 1.00]           NA         NA        NA
#> 5  textual      x5  last 1.11 [0.98, 1.24]           NA         NA        NA
#> 6  textual      x6  last 0.93 [0.82, 1.03]           NA         NA        NA
#> 7    speed      x7  last 1.00 [1.00, 1.00]           NA         NA        NA
#> 8    speed      x8  last 1.18 [0.86, 1.50]           NA         NA        NA
#> 9    speed      x9  last 1.08 [0.79, 1.38]           NA         NA        NA
#> 10      x1      x1  both 0.55 [0.33, 0.77]           NA         NA        NA
#> 11      x2      x2  both 1.13 [0.93, 1.33]           NA         NA        NA
#> 12      x3      x3  both 0.84 [0.67, 1.02]           NA         NA        NA
#> 13      x4      x4  both 0.37 [0.28, 0.46]           NA         NA        NA
#> 14      x5      x5  both 0.45 [0.33, 0.56]           NA         NA        NA
#> 15      x6      x6  both 0.36 [0.27, 0.44]           NA         NA        NA
#> 16      x7      x7  both 0.80 [0.64, 0.96]           NA         NA        NA
#> 17      x8      x8  both 0.49 [0.34, 0.63]           NA         NA        NA
#> 18      x9      x9  both 0.57 [0.43, 0.70]           NA         NA        NA
#> 19  visual  visual  both 0.81 [0.52, 1.09]           NA         NA        NA
#> 20 textual textual  both 0.98 [0.76, 1.20]           NA         NA        NA
#> 21   speed   speed  both 0.38 [0.21, 0.55]           NA         NA        NA
#> 22  visual textual  none 0.41 [0.26, 0.55]           NA         NA        60
#> 23  visual   speed  none 0.26 [0.15, 0.37]           NA         NA        60
#> 24 textual   speed  none 0.17 [0.08, 0.27]           NA         NA        60

We can do the same for the nodes, but at this time, the mean structure is not part of the model, so table_results() will not retrieve any parameter estimates for the nodes. Unless label = NULL, get_nodes() will use the node name as a label when no further information is available.

We can re-run the model with a mean structure to get more information about the nodes:

fit <- cfa(HS.model, data=HolzingerSwineford1939, meanstructure = TRUE)

Running get_nodes() now uses the estimated means in the label:

get_nodes(fit)
#>       name shape         label
#> 1    speed  oval   speed\n0.00
#> 2  textual  oval textual\n0.00
#> 3   visual  oval  visual\n0.00
#> 4       x1  rect   x1\n4.94***
#> 5       x2  rect   x2\n6.09***
#> 6       x3  rect   x3\n2.25***
#> 7       x4  rect   x4\n3.06***
#> 8       x5  rect   x5\n4.34***
#> 9       x6  rect   x6\n2.19***
#> 10      x7  rect   x7\n4.19***
#> 11      x8  rect   x8\n5.53***
#> 12      x9  rect   x9\n5.37***

These labels can be further customized as shown below:

get_nodes(fit, label = paste0(name, "\n", est, " ", confint))
#>       name shape                      label
#> 1    speed  oval   speed\n0.00 [0.00, 0.00]
#> 2  textual  oval textual\n0.00 [0.00, 0.00]
#> 3   visual  oval  visual\n0.00 [0.00, 0.00]
#> 4       x1  rect      x1\n4.94 [4.80, 5.07]
#> 5       x2  rect      x2\n6.09 [5.96, 6.22]
#> 6       x3  rect      x3\n2.25 [2.12, 2.38]
#> 7       x4  rect      x4\n3.06 [2.93, 3.19]
#> 8       x5  rect      x5\n4.34 [4.19, 4.49]
#> 9       x6  rect      x6\n2.19 [2.06, 2.31]
#> 10      x7  rect      x7\n4.19 [4.06, 4.31]
#> 11      x8  rect      x8\n5.53 [5.41, 5.64]
#> 12      x9  rect      x9\n5.37 [5.26, 5.49]

Auxiliary columns

Instead of customizing the node and edge labels in the call to get_nodes() and get_edges(), it is also possible to request auxiliary columns, and use these to customize the labels (or other properties of the plot) later. Below is an example for get_edges(), but the same mechanism applies to get_nodes():

get_edges(fit, label = "est", columns = "pval")
#>       from      to arrow label pval connect_from connect_to curvature
#> 1   visual      x1  last  1.00 <NA>           NA         NA        NA
#> 2   visual      x2  last  0.55 0.00           NA         NA        NA
#> 3   visual      x3  last  0.73 0.00           NA         NA        NA
#> 4  textual      x4  last  1.00 <NA>           NA         NA        NA
#> 5  textual      x5  last  1.11 0.00           NA         NA        NA
#> 6  textual      x6  last  0.93 0.00           NA         NA        NA
#> 7    speed      x7  last  1.00 <NA>           NA         NA        NA
#> 8    speed      x8  last  1.18 0.00           NA         NA        NA
#> 9    speed      x9  last  1.08 0.00           NA         NA        NA
#> 10      x1      x1  both  0.55 0.00           NA         NA        NA
#> 11      x2      x2  both  1.13 0.00           NA         NA        NA
#> 12      x3      x3  both  0.84 0.00           NA         NA        NA
#> 13      x4      x4  both  0.37 0.00           NA         NA        NA
#> 14      x5      x5  both  0.45 0.00           NA         NA        NA
#> 15      x6      x6  both  0.36 0.00           NA         NA        NA
#> 16      x7      x7  both  0.80 0.00           NA         NA        NA
#> 17      x8      x8  both  0.49 0.00           NA         NA        NA
#> 18      x9      x9  both  0.57 0.00           NA         NA        NA
#> 19  visual  visual  both  0.81 0.00           NA         NA        NA
#> 20 textual textual  both  0.98 0.00           NA         NA        NA
#> 21   speed   speed  both  0.38 0.00           NA         NA        NA
#> 22  visual textual  none  0.41 0.00           NA         NA        60
#> 23  visual   speed  none  0.26 0.00           NA         NA        60
#> 24 textual   speed  none  0.17 0.00           NA         NA        60

We can use this information to manually construct labels:

get_edges(fit, label = "est", columns = c("est", "pval")) %>%
  within({ label[!is.na(pval)] = paste0(est[!is.na(pval)], ", p = ", pval[!is.na(pval)])})
#>       from      to arrow          label  est pval connect_from connect_to
#> 1   visual      x1  last           1.00 1.00 <NA>           NA         NA
#> 2   visual      x2  last 0.55, p = 0.00 0.55 0.00           NA         NA
#> 3   visual      x3  last 0.73, p = 0.00 0.73 0.00           NA         NA
#> 4  textual      x4  last           1.00 1.00 <NA>           NA         NA
#> 5  textual      x5  last 1.11, p = 0.00 1.11 0.00           NA         NA
#> 6  textual      x6  last 0.93, p = 0.00 0.93 0.00           NA         NA
#> 7    speed      x7  last           1.00 1.00 <NA>           NA         NA
#> 8    speed      x8  last 1.18, p = 0.00 1.18 0.00           NA         NA
#> 9    speed      x9  last 1.08, p = 0.00 1.08 0.00           NA         NA
#> 10      x1      x1  both 0.55, p = 0.00 0.55 0.00           NA         NA
#> 11      x2      x2  both 1.13, p = 0.00 1.13 0.00           NA         NA
#> 12      x3      x3  both 0.84, p = 0.00 0.84 0.00           NA         NA
#> 13      x4      x4  both 0.37, p = 0.00 0.37 0.00           NA         NA
#> 14      x5      x5  both 0.45, p = 0.00 0.45 0.00           NA         NA
#> 15      x6      x6  both 0.36, p = 0.00 0.36 0.00           NA         NA
#> 16      x7      x7  both 0.80, p = 0.00 0.80 0.00           NA         NA
#> 17      x8      x8  both 0.49, p = 0.00 0.49 0.00           NA         NA
#> 18      x9      x9  both 0.57, p = 0.00 0.57 0.00           NA         NA
#> 19  visual  visual  both 0.81, p = 0.00 0.81 0.00           NA         NA
#> 20 textual textual  both 0.98, p = 0.00 0.98 0.00           NA         NA
#> 21   speed   speed  both 0.38, p = 0.00 0.38 0.00           NA         NA
#> 22  visual textual  none 0.41, p = 0.00 0.41 0.00           NA         NA
#> 23  visual   speed  none 0.26, p = 0.00 0.26 0.00           NA         NA
#> 24 textual   speed  none 0.17, p = 0.00 0.17 0.00           NA         NA
#>    curvature
#> 1         NA
#> 2         NA
#> 3         NA
#> 4         NA
#> 5         NA
#> 6         NA
#> 7         NA
#> 8         NA
#> 9         NA
#> 10        NA
#> 11        NA
#> 12        NA
#> 13        NA
#> 14        NA
#> 15        NA
#> 16        NA
#> 17        NA
#> 18        NA
#> 19        NA
#> 20        NA
#> 21        NA
#> 22        60
#> 23        60
#> 24        60

The section on Visual aspects demonstrates how to use these auxiliary columns to customize visual properties of the graph.

Optional step 5: accessing graph data before plotting

One important feature of tidySEM graphing is that the data used to compose the plot can be conveniently accessed an modified before plotting. First, use prepare_graph() to assign the plot data to an object.

graph_data <- prepare_graph(model = fit, layout = lay)

Optional step 6: Access the nodes and edges

The nodes and edges can be examined using nodes(graph_data) and edges(graph_data):

nodes(graph_data)
#>       name shape   label  x y node_xmin node_xmax node_ymin node_ymax
#> 1    speed  oval    0.00 14 4      13.5      14.5       3.5       4.5
#> 2  textual  oval    0.00 10 4       9.5      10.5       3.5       4.5
#> 3   visual  oval    0.00  6 4       5.5       6.5       3.5       4.5
#> 4       x1  rect 4.94***  2 2       1.4       2.6       1.6       2.4
#> 5       x2  rect 6.09***  4 2       3.4       4.6       1.6       2.4
#> 6       x3  rect 2.25***  6 2       5.4       6.6       1.6       2.4
#> 7       x4  rect 3.06***  8 2       7.4       8.6       1.6       2.4
#> 8       x5  rect 4.34*** 10 2       9.4      10.6       1.6       2.4
#> 9       x6  rect 2.19*** 12 2      11.4      12.6       1.6       2.4
#> 10      x7  rect 4.19*** 14 2      13.4      14.6       1.6       2.4
#> 11      x8  rect 5.53*** 16 2      15.4      16.6       1.6       2.4
#> 12      x9  rect 5.37*** 18 2      17.4      18.6       1.6       2.4
edges(graph_data)
#>       from      to   label arrow curvature connect_from connect_to
#> 1   visual      x1    1.00  last        NA         left      right
#> 2   visual      x2 0.55***  last        NA       bottom      right
#> 3   visual      x3 0.73***  last        NA       bottom        top
#> 4  textual      x4    1.00  last        NA       bottom      right
#> 5  textual      x5 1.11***  last        NA       bottom        top
#> 6  textual      x6 0.93***  last        NA       bottom       left
#> 7    speed      x7    1.00  last        NA       bottom        top
#> 8    speed      x8 1.18***  last        NA       bottom       left
#> 9    speed      x9 1.08***  last        NA        right       left
#> 10      x1      x1 0.55***  both        NA       bottom     bottom
#> 11      x2      x2 1.13***  both        NA       bottom     bottom
#> 12      x3      x3 0.84***  both        NA       bottom     bottom
#> 13      x4      x4 0.37***  both        NA       bottom     bottom
#> 14      x5      x5 0.45***  both        NA       bottom     bottom
#> 15      x6      x6 0.36***  both        NA       bottom     bottom
#> 16      x7      x7 0.80***  both        NA       bottom     bottom
#> 17      x8      x8 0.49***  both        NA       bottom     bottom
#> 18      x9      x9 0.57***  both        NA       bottom     bottom
#> 19  visual  visual 0.81***  both        NA        right      right
#> 20 textual textual 0.98***  both        NA         left       left
#> 21   speed   speed 0.38***  both        NA         left       left
#> 22  visual textual 0.41***  none        60          top        top
#> 23  visual   speed 0.26***  none        60          top        top
#> 24 textual   speed 0.17***  none        60          top        top

Optional step 7: Modify the nodes and edges

At this stage, we may want to improve the basic plot slightly. The functions nodes(graph_data) <- ... and edges(graph_data) <- ... can be used to modify the nodes and edges. These functions pair well with the general ‘tidyverse’ workflow. For example, we might want to print node labels for latent variables in Title Case instead of just using the variable names:

library(dplyr)
library(stringr)
nodes(graph_data) <- nodes(graph_data) %>%
  mutate(label = str_to_title(label))

Now, for the edges, we see that the default edging algorithm has connected some nodes side-to-side (based on the smallest possible Euclidian distance). However, in this simple graph, it makes more sense to connect all nodes top-to-bottom - except for the latent variable covariances. We can use the same conditional replacement for the edges:

edges(graph_data) %>%
  mutate(connect_from = replace(connect_from, is.na(curvature), "bottom")) %>%
  mutate(connect_to = replace(connect_to, is.na(curvature), "top")) -> edges(graph_data)

Plot the customized graph

We can plot a customized graph using plot(graph_data); a generic plot method for sem_graph objects:

plot(graph_data)

Connecting nodes

The functions graph_sem() and prepare_graph() will always try to connect nodes in an aesthetically pleasing way. To do this, they connect nodes by one of the four sides (top, bottom, left and right), based on the shortest distance between two nodes. Alternatively, users can specify a value to the angle parameter. This parameter connects nodes top-to-bottom when they are within angle degrees above and below each other. Remaining nodes are connected side-to-side. Thus, by increasing angle to a larger number (up to 180 degrees), we can also ensure that all nodes are connected top to bottom:

graph_sem(model = fit, layout = lay, angle = 170)

Visual aspects

The visual aspects of graphs created using graph_sem() and prepare_graph() can be easily customized (see ?graph_sem()). These functions construct a graph based on a data.frame of nodes, and a data.frame of edges. The visual aspects can be customized by adding extra columns to those data.frames.

Visual aspects of edges

Edges have the following aesthetics (see ?geom_path()):

linetype: The type of line, e.g., continuous (linetype = 1 or dashed linetype = 2)
colour or color: The color of the line (e.g., colour = "red" or colour = "blue")
size: The size of the line (e.g., size = 1 or size = 2)
alpha: The transparency of the line (e.g., alpha = 1 or alpha = .2)

These aesthetics can be customized by adding columns to the edges data.frame, whose names correspond to the aesthetics. For example:

edg <- data.frame(from = "x",
                  to = "y",
                  linetype = 2,
                  colour = "red",
                  size = 2,
                  alpha = .5)

graph_sem(edges = edg, layout = get_layout("x", "y", rows = 1))

Visual aspects of nodes

Nodes have the following aesthetics (see ?geom_polygon()):

linetype: The type of line surrounding the node, e.g., continuous (linetype = 1 or dashed linetype = 2)
colour or color: The color of the line surrounding the node (e.g., colour = "red" or colour = "blue")
fill: The interior color of the node (e.g., colour = "red" or colour = "blue")
size: The size of the line (e.g., size = 1 or size = 2)
alpha: The transparency of the node (e.g., alpha = 1 or alpha = .2)

These, too, can be appended as extra columns to the nodes data.frame:

edg <- data.frame(from = "x",
                  to = "y")
nod <- data.frame(name = c("x", "y"),
                    shape = c("rect", "oval"),
                    linetype = c(2, 2),
                    colour = c("blue", "blue"),
                    fill = c("blue", "blue"),
                    size = c(2, 2),
                    alpha = .5)
graph_sem(edges = edg, nodes = nod, layout = get_layout("x", "y", rows = 1))

These aesthetics can also be passed to the sem_graph object after preparing the data, for example, for highlighting a specific model element, such as the low factor loading for x2 on visual in the CFA example from before:

edges(graph_data) %>%
  mutate(colour = "black") %>%
  mutate(colour = replace(colour, from == "visual" & to == "x2", "red")) %>%
  mutate(linetype = 1) %>%
  mutate(linetype = replace(linetype, from == "visual" & to == "x2", 2)) %>%
  mutate(alpha = 1) %>%
  mutate(alpha = replace(alpha, from == "visual" & to == "x2", .5)) -> edges(graph_data)
plot(graph_data)

Visual aspects of edge labels

Like nodes and edges, edge labels can be customized. Labels have the same aesthetics as the ggplot function geom_label() (see ?geom_label). However, to disambiguate them from the edge aesthetics, all label aesthetics are prefaced by "label_":

label_colour or label_color: The color of the label text (e.g., label_colour = "red")
label_fill: The color of the rectangle behind the label text, ( e.g., label_fill = "blue")
label_size: The font size of the label (e.g., label_size = 4)
label_alpha: The transparency of the label (e.g., alpha = 1 or alpha = .2)
label_family: The font family of the label text. By default, three are available: "serif", "sans" and "mono"
label_fontface: The face of the label text: "plain", "bold", "italic", or "bold.italic"
label_hjust: Horizontal justification of label text; either a string ("left", "center", "right") or a number between 0 and 1 (left = 0, center = 0.5, right = 1)
label_vjust: Vertical justification of label text; either a string ("top", "middle", "bottom") or a number between 0 and 1 (top = 1, middle = 0.5, bottom = 0)
label_lineheight: Line height for multiline text
label_location: Where along the edge to plot the label; a number between 0 (beginning) and 1 (end)

edg <- data.frame(from = "x",
                  to = "y",
                  label = "text",
                  label_colour = "blue",
                  label_fill = "red",
                  label_size = 6,
                  label_alpha = .5,
                  label_family = "mono",
                  label_fontface = "bold",
                  label_hjust = "left",
                  label_vjust = "top",
                  label_lineheight = 1.5,
                  label_location = .2
                  )

graph_sem(edges = edg, layout = get_layout("x", "y", rows = 1))

Using auxiliary columns to customize visual aspects

As explained before, the functions get_edges() and get_nodes(), which are used in the default arguments of prepare_graph() and graph_sem(), have an argument columns which adds auxiliary columns to the resulting data.frames. This can be used to customize visual aspects of the edges and nodes based on, for example, the value of the estimates:

fit <- sem("mpg ~ cyl
           mpg ~ am", data = mtcars)

p <- prepare_graph(fit,
                   edges = get_edges(fit, columns = "est"),
                   fix_coord = TRUE)
edges(p) %>%
  mutate(color = "black",
         color = replace(color, arrow == "last" & as.numeric(est) < 0, "red"),
         color = replace(color, arrow == "last" & as.numeric(est) > 0, "green")) -> edges(p)

plot(p)