qqPlot in a grid

Question

I would like to have some plots arranged in a grid, and one of these plots is a qq-plot using the package car. The following

library(car)
library(ggplot2)
library(gridExtra)

n <- 100
df <- data.frame( x=rnorm(n) )
df <- transform(df, y=3*x-1)
p <- ggplot(data=df,aes(x=x,y=y)) + geom_point()
q <- qqPlot(df$x)
grid.arrange(p,q,ncol=2)

issues an error since qqPlot() always plots and returns NULL.

Answer 1

Based on the answers to the question Combine base and ggplot graphics in R figure window, I could find a solution

library(ggplot2)
library(grid)

n <- 100
df <- data.frame( x=rnorm(n) )
df <- transform(df, y=3*x-1)
par(mfrow=c(1,2))
p <- ggplot(data=df,aes(x=x,y=y)) + geom_point()
qqPlot(df$x)
vp.right <- viewport(height=unit(.5,"npc"), width=unit(0.5,"npc"),
                     y=0.5, x=0.7)
print(p, vp=vp.right)