Make a rectangular legend, with rows and columns labeled, in grid

Question

I\'ve got a ggplot where I\'m mapping factors to both fill and alpha, like this:

set.seed(47)
the_data <- data.frame(value = rpois(6, lambda=20),
                 


        

Answer 1

@Henrik

This might be a little easier,

g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)

leg1 <- gtable_filter(g1, "guide-box") 
leg2 <- gtable_filter(g2, "guide-box") 
leg <- gtable:::cbind_gtable(leg1[["grobs"]][[1]],  leg2[["grobs"]][[1]], "first")

g1$grobs[g1$layout$name == "guide-box"][[1]] <- leg
g1$widths[max(subset(g1$layout, name == "guide-box")[["r"]])] <- list(leg1$width + leg2$width)

grid.newpage()
grid.draw(g1)