library(ggplot2)
library(gridExtra)
dsamp <- diamonds[sample(nrow(diamonds), 1000), ]

p1 <- qplot(carat, price, data=dsamp, colour=clarity)
p2 <- qplot(carat, price, data=dsamp, colour=clarity, geom="path")

g_legend<-function(p){
tmp <- ggplotGrob(p)
leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
legend <- tmp$grobs[[leg]]
return(legend)}

legend <- g_legend(p1)
lwidth <- sum(legend$width)

## using grid.arrange for convenience
## could also manually push viewports
grid.arrange(arrangeGrob(p1 + theme(legend.position="none"),
                                    p2 + theme(legend.position="none"),
                                    main ="this is a title",
                                    left = "This is my global Y-axis title"), legend,
                 widths=unit.c(unit(1, "npc") - lwidth, lwidth), nrow=1)

p1 <- qplot(carat, price*0.001, data=dsamp, colour=clarity)
p2 <- qplot(carat, price, data=dsamp, colour=clarity, geom="path")

g_legend<-function(p){
tmp <- ggplotGrob(p)
leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
legend <- tmp$grobs[[leg]]
return(legend)}

legend <- g_legend(p1)
lwidth <- sum(legend$width)

## using grid.arrange for convenience
## could also manually push viewports
grid.arrange(arrangeGrob(p1 + theme(legend.position="none"),
                                    p2 + theme(legend.position="none"),
                                    main ="this is a title",
                                    left = "This is my global Y-axis title"), legend,
                 widths=unit.c(unit(1, "npc") - lwidth, lwidth), nrow=1)