Grid.arrange forestplot objects

Question

I have a couple of forest-plots made using the R package forestplot. I would like to grid.arrange them - but it seems this doesn't work easily.

Example:

library(forestplot)
# A basic example, create some fake data
row_names <- list(list("test = 1", expression(test >= 2)))
test_data <- data.frame(coef=c(1.59, 1.24),
                    low=c(1.4, 0.78),
                    high=c(1.8, 1.55))

forestplot(row_names,
       test_data$coef,
       test_data$low,
       test_data$high,
       zero = 1,
       cex  = 2,
       lineheight = "auto",
       xlab = "Lab axis txt")

Ok this will draw a plot. Now suppose I want to capture that into an object plot it side by side with another plot:

fp1 <- forestplot(row_names,
       test_data$coef,
       test_data$low,
       test_data$high,
       zero = 1,
       cex  = 2,
       lineheight = "auto",
       xlab = "Lab axis txt")

The following throws an error:

> grid.arrange(fp1, fp1)
Hit <Return> to see next plot: 
Error in gList(list(path = "GRID.VP.7537", name = "arrange.1-1-1-1", n = 2L,  : 
only 'grobs' allowed in "gList"

So cleary fp1 is not a grob - but how do I achieve this by other means ?

Answer 1

See the second example in the help pages ?forestplot. which shows how to do this.

---

forestplot doesn't seem to return the plot: look at str(fp1).

A couple of options is use grid to create the plotting space (v1), or else capture the plot and then combine (v2).

---

v1 using grid

library(grid)
library(forestplot)

# Create 2 rows by one columns viewport
grid.newpage()
pushViewport(viewport(layout=grid.layout(2, 1)))

# Plot in viewport position 1x1
pushViewport(viewport(layout.pos.row=1, layout.pos.col=1))
forestplot(row_names,
       test_data$coef,
       test_data$low,
       test_data$high,
       zero = 1,
       cex  = 2,
       lineheight = "auto",
       xlab = "Lab axis txt")
upViewport(1)

# Plot in viewport position 2x1
pushViewport(viewport(layout.pos.row=2, layout.pos.col=1))
forestplot(row_names,
       test_data$coef,
       test_data$low,
       test_data$high,
       zero = 1,
       cex  = 2,
       lineheight = "auto",
       xlab = "Lab axis txt", 
       new_page = FALSE)
upViewport(1)

---

v2, capture the plot

fp1 <- grid.grabExpr(print(forestplot(row_names,
       test_data$coef,
       test_data$low,
       test_data$high,
       zero = 1,
       cex  = 2,
       lineheight = "auto",
       xlab = "Lab axis txt")))

gridExtra::grid.arrange(fp1, fp1)