multipanel ggplot from a list with grid_arrange_shared_legend - r

Question

I'm trying to make my multipanel ggplot with a shared legend more flexible in a ShinyApp by allowing the user to choose how many panels to plot.
Currently my code writes out the panel objects 1 at a time like this.
grid_arrange_shared_legend(p1,p2,p3,p4, ncol = 4, nrow = 1)
I do not fully understand why I can not find a way to tell the grid_arrange_shared_legend to accept a list of plots (list object) rather than writing them out 1 after the other.
It throws this error:
Error in UseMethod("ggplot_build") :
no applicable method for 'ggplot_build' applied to an object of class "NULL"
library(ggplot2)
library(lemon)
plotlist <- list()
dsamp <- diamonds[sample(nrow(diamonds), 300), ]
plotlist$p1 <- qplot(carat, price, data = dsamp, colour = clarity)
plotlist$p2 <- qplot(cut, price, data = dsamp, colour = clarity)
plotlist$p3 <- qplot(color, price, data = dsamp, colour = clarity)
plotlist$p4 <- qplot(depth, price, data = dsamp, colour = clarity)
grid_arrange_shared_legend(plotlist, ncol = 4, nrow = 1)
with the use of a list, it would not matter how many plots are in the list, and I would calculate ncol or nrow based on the length of the list...

Answer 1

My homebrew version of the function gets that by adding a plotlist parameter, and adding the plots <- c(list(...), plotlist) line as the first line of code. That way it can take both a list of plots or separate plot objects.
grid_arrange_shared_legend_plotlist <- function(...,
plotlist=NULL,
ncol = length(list(...)),
nrow = NULL,
position = c("bottom", "right")) {
plots <- c(list(...), plotlist)
if (is.null(nrow)) nrow = ceiling(length(plots)/ncol)
position <- match.arg(position)
g <- ggplotGrob(plots[[1]] + theme(legend.position = position))$grobs
legend <- g[[which(sapply(g, function(x) x$name) == "guide-box")]]
lheight <- sum(legend$height)
lwidth <- sum(legend$width)
gl <- lapply(plots, function(x) x + theme(legend.position="none"))
gl <- c(gl, ncol = ncol, nrow = nrow)
combined <- switch(position,
"bottom" = arrangeGrob(do.call(arrangeGrob, gl),
legend,
ncol = 1,
heights = unit.c(unit(1, "npc") - lheight, lheight)),
"right" = arrangeGrob(do.call(arrangeGrob, gl),
legend,
ncol = 2,
widths = unit.c(unit(1, "npc") - lwidth, lwidth)))
grid.newpage()
grid.draw(combined)
# return gtable invisibly
invisible(combined)
}
Using your example:
library(gridExtra)
library(grid)
library(ggplot2)
plots <- list()
dsamp <- diamonds[sample(nrow(diamonds), 300), ]
plots$p1 <- qplot(carat, price, data = dsamp, colour = clarity)
plots$p2 <- qplot(cut, price, data = dsamp, colour = clarity)
plots$p3 <- qplot(color, price, data = dsamp, colour = clarity)
plots$p4 <- qplot(depth, price, data = dsamp, colour = clarity)
grid_arrange_shared_legend_plotlist(plotlist = plots, ncol = 4)

Answer 2

The ugly text string paste solution:
Since the answers provided do not seem to work, or are not suitable (rebuilding an entirely different set of plots than the list of plot objects I already have from extensive code, I played around a bit with eval(parse(text = ....) and paste0 to dynamically generate a text string that ends up being the fully written out code (which works) without actually writing it out
nplots = 4
nrow = 2
ncol = ceiling(nplots/nrow)
eval(parse( text = paste0("grid_arrange_shared_legend(", paste0("plotlist", "[[", c(1:nplots), "]]", sep = '', collapse = ','), ",ncol =", ncol, ",nrow =", nrow, ", position = 'right', top=grid::textGrob('My title', gp=grid::gpar(fontsize=18)))", sep = '')))
which produces:
[1]
"grid_arrange_shared_legend(plotlist[[1]],plotlist[[2]],plotlist[[3]],plotlist[[4]],ncol
=2,nrow =2, position = 'right', top=grid::textGrob('My title', gp=grid::gpar(fontsize=18)))"