Global legend using grid.arrange (gridExtra) and lattice based plots

Question

I am producing four plots using xyplot (lattice) and further combine them with grid.arrange (gridExtra).

I would like to obtain a graph with a common global legend.

Answer 1

One possible solution is to use ggplot, hinted here.

my.cols <- 1:3

my.grid.layout <- rbind(c(1,2),
                        c(3,3))

g_legend<-function(a.gplot){
    tmp <- ggplot_gtable(ggplot_build(a.gplot))
    leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
    legend <- tmp$grobs[[leg]]
    return(legend)
  }

legend.plot <- ggplot(iris, aes(x=Petal.Length, y=Sepal.Width,colour=Species)) +
                                  geom_line(size=1) + # legend should show lines, not points or rects ...
                                  theme(legend.position="right", legend.background = element_rect(colour = "black"),
                                        legend.key = element_rect(fill = "white")) + # position, box and background colour of legend
                                  scale_color_manual(values=my.cols, name = "Categories") + # manually insert colours as used in corresponding xyplot
                                  guides(colour = guide_legend(reverse=T)) # inverts order of colours in legend

mylegend <- g_legend(legend.plot)
plot1 <- xyplot(Sepal.Width ~ Petal.Length, groups = Species, data = iris, type = 'l',
                par.settings = simpleTheme(col=my.cols))
plot2 <- xyplot(Sepal.Length ~ Petal.Length, groups = Species, data = iris, type = 'l',
                par.settings = simpleTheme(col=my.cols))


grid.arrange(plot1,plot2,mylegend,layout_matrix=my.grid.layout,             
         top=textGrob(gp=gpar(col='black',fontsize=20),"Some useless example"))