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Plot Surface for Interaction Effects

plot_surface.Rd

Generates a 3D surface plot to visualize the interaction effect of two variables (x and z) on an outcome (y) using parameter estimates from a supported model object (e.g., lavaan or modsem). The function allows specifying ranges for x and z in standardized z-scores, which are then transformed back to the original scale based on their means and standard deviations.

Usage

plot_surface(
  x,
  z,
  y,
  model,
  min_x = -3,
  max_x = 3,
  min_z = -3,
  max_z = 3,
  standardized = FALSE,
  detail = 0.01,
  xz = NULL,
  colorscale = "Viridis",
  reversescale = FALSE,
  showscale = TRUE,
  cmin = NULL,
  cmax = NULL,
  surface_opacity = 1,
  grid = FALSE,
  grid_nx = 12,
  grid_ny = 12,
  grid_color = "rgba(0,0,0,0.45)",
  group = NULL,
  ...
)

Arguments

x

A character string specifying the name of the first predictor variable.

z

A character string specifying the name of the second predictor variable.

y

A character string specifying the name of the outcome variable.

model

A model object of class modsem_pi, modsem_da, modsem_mplus, or lavaan. The model should include paths for the predictors (x, z, and xz) to the outcome (y).

min_x

Numeric. Minimum value of x in z-scores. Default is -3.

max_x

Numeric. Maximum value of x in z-scores. Default is 3.

min_z

Numeric. Minimum value of z in z-scores. Default is -3.

max_z

Numeric. Maximum value of z in z-scores. Default is 3.

standardized

Should coefficients be standardized beforehand?

detail

Numeric. Step size for the grid of x and z values, determining the resolution of the surface. Smaller values increase plot resolution. Default is 1e-2.

xz

Optional. A character string or vector specifying the interaction term between x and z. If NULL, the interaction term is constructed as paste(x, z, sep = ":") and adjusted for specific model classes.

colorscale

Character or list. Colorscale used to color the surface. - Default is "Viridis", which matches the classic Plotly default. - Can be a built-in palette name (e.g., "Greys", "Plasma", "Turbo"), or a custom two-column list with numeric stops (0–1) and color codes. Example custom scale: list(c(0, "white"), c(1, "black")) for a black-and-white gradient.

reversescale

Logical. If TRUE, reverses the color mapping so that low values become high colors and vice versa. Default is FALSE.

showscale

Logical. If TRUE, displays the colorbar legend alongside the plot. Default is TRUE.

cmin

Numeric or NULL. The minimum value for the colorscale mapping. If NULL, the minimum of the data (proj_y) is used automatically. Use this to standardize the color range across multiple plots.

cmax

Numeric or NULL. The maximum value for the colorscale mapping. If NULL, the maximum of the data (proj_y) is used automatically. Use this to standardize the color range across multiple plots.

surface_opacity

Numeric (0–1). Controls the opacity of the surface. - 1 = fully opaque (default) - 0 = fully transparent Useful when overlaying multiple surfaces or highlighting gridlines.

grid

Logical. If TRUE, draws gridlines (wireframe) directly on the surface using Plotly's contour features. Default is FALSE.

grid_nx

Integer. Approximate number of gridlines to draw along the **x-axis** direction when grid = TRUE. Higher values create a denser grid. Default is 12.

grid_ny

Integer. Approximate number of gridlines to draw along the **y-axis** direction when grid = TRUE. Higher values create a denser grid. Default is 12.

grid_color

Character. Color of the gridlines drawn on the surface. Must be a valid CSS color string, including rgba() for transparency. - Default is "rgba(0,0,0,0.45)" (semi-transparent black). Example: "rgba(255,255,255,0.8)" for semi-transparent white lines.

group

Which group to create surface plot for. Only relevant for multigroup models. Must be an integer index, representing the nth group.

...

Additional arguments passed to plotly::plot_ly.

Value

A plotly surface plot object displaying the predicted values of y across the grid of x and z values. The color bar shows the values of y.

Details

The input min_x, max_x, min_z, and max_z define the range of x and z values in z-scores. These are scaled by the standard deviations and shifted by the means of the respective variables, obtained from the model parameter table. The resulting surface shows the predicted values of y over the grid of x and z.

The function supports models of class modsem (with subclasses modsem_pi, modsem_da, modsem_mplus) and lavaan. For lavaan models, it is not designed for multigroup models, and a warning will be issued if multiple groups are detected.

Note

The interaction term (xz) may need to be manually specified for some models. For non-lavaan models, interaction terms may have their separator (:) removed based on circumstances.

Examples

m1 <- "
# Outer Model
  X =~ x1 + x2 + x3
  Z =~ z1 + z2 + z3
  Y =~ y1 + y2 + y3

# Inner model
  Y ~ X + Z + X:Z
"
est1 <- modsem(m1, data = oneInt)
plot_surface("X", "Z", "Y", model = est1)



# \dontrun{
tpb <- "
# Outer Model (Based on Hagger et al., 2007)
  ATT =~ att1 + att2 + att3 + att4 + att5
  SN =~ sn1 + sn2
  PBC =~ pbc1 + pbc2 + pbc3
  INT =~ int1 + int2 + int3
  BEH =~ b1 + b2

# Inner Model (Based on Steinmetz et al., 2011)
  INT ~ ATT + SN + PBC
  BEH ~ INT + PBC
  BEH ~ PBC:INT
"

est2 <- modsem(tpb, TPB, method = "lms", nodes = 32)
plot_surface(x = "INT", z = "PBC", y = "BEH", model = est2)


# }