Using car::scatter3d(), I am trying to create a 3D figure with a regression surface indicating an interaction between a categorical and a continuous variable. Partly following the code here, I obtained a figure below.
The figure is obviously wrong in that the regression surface does not reach one of the values of the categorical variable. The problem perhaps lies in the use of the rgl::persp3d() (the last block of the code below), but I have not been able to identify what exactly I'm doing wrongly. Could someone let me know what I'm missing and how to fix the problem?
library(rgl)
library(car)
n <- 100
set.seed(1)
x <- runif(n, 0, 10)
set.seed(1)
z <- sample(c(0, 1), n, replace = TRUE)
set.seed(1)
y <- 0.5 * x + 0.1 * z + 0.3 * x * z + rnorm(n, sd = 1.5)
d <- data.frame(x, z, y)
scatter3d(y ~ x + z, data = d,
xlab = "continuous", zlab = "categorical", ylab = "outcome",
residuals = FALSE, surface = FALSE
)
d2 <- d
d2$x <- d$x / (max(d$x) - min(d$x))
d2$y <- d$y / (max(d$y) - min(d$y))
mod <- lm(y ~ x * z, data = d2)
grd <- expand.grid(x = unique(d2$x), z = unique(d2$z))
grd$pred <- predict(mod, newdata = grd)
grd <- grd[order(grd$z, grd$x), ]
# The problem is likely to lie somewhere below.
persp3d(x = unique(grd$x), y = unique(grd$z),
z = matrix(grd$pred, length(unique(grd$z)), length(unique(grd$x))),
alpha = 0.5,
col = "blue",
add = TRUE,
xlab = "", ylab = "", zlab = ""
)
I prefer sticking to car::scatter3d() in drawing the original graph because I already made several figures with car::scatter3d() and want to make this figure consistent with them as well.
