Setting the color label and varying the color palette in qplot

Question

1. Using the code below, i can set the labels for x and y axis, but can't set the label for color that is cyl here. The documentation provides no way around.

   qplot(mpg, wt, data=mtcars, colour=cyl,xlab="MPG",ylab="WT")
   

2. How can I vary the color palette herein qplot? So, I wish to do something like in the code below:

   x <- runif(100)
   y<-runif(100)
   time<-runif(100)  
   pal <- colorRampPalette(c('white','black'))
   cols <- pal(10)[as.numeric(cut(time,breaks = 10))]
   plot(x,y,pch=19,col = cols)
   

Answer 1

You can use scale_colour_continuous for both tasks.

library(ggplot2)
qplot(mpg, wt, data = mtcars, colour = cyl, xlab = "MPG", ylab = "WT") +
  scale_colour_continuous(name = "Cylinders", low = "white", high = "black")

Here, the name parameter is the label for the colour scale. The parameters low and high denote the lower and upper limits of the continuous colour scale.

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If you want to specify a continuous colour scale with three colours, you can use scale_colour_gradient2:

qplot(mpg, wt, data = mtcars, colour = cyl, xlab = "MPG", ylab = "WT") +
  scale_colour_gradient2(name = "Cylinders", midpoint = median(mtcars$cyl),
                         low = "red", mid = "green", high = "black")