testdata = data.frame(var1 = seq(0,10,by=1), var2 = runif(11),
var3 = runif(11, min = 100, max = 500))

renderPlot({grid.arrange(
ggplot(data = testdata, aes(x = var1, y = var2))
+ geom_line(colour = "blue") + xlab(NULL),
ggplot(data = testdata, aes(x = var1, y = var3)) + geom_line(colour = "red"))})