You want to represent a third continuous variable using color or size.

Map the continuous variable to size or colour. In the heightweight data set, there are many columns, but we’ll only use four of them in this example:

library(gcookbook) # For the data set

# List the four columns we'll use
heightweight[, c("sex", "ageYear", "heightIn", "weightLb")]

 sex ageYear heightIn weightLb
   f   11.92     56.3     85.0
   f   12.92     62.3    105.0
   f   12.75     63.3    108.0
...
   m   13.92     62.0    107.5
   m   12.58     59.3     87.0

The basic scatter plot in Making a Basic Scatter Plot shows the relationship between the continuous variables ageYear and heightIn. To represent a third continuous variable, weightLb, we must map it to another aesthetic property. We can map it to colour or size, as shown in Figure 5-9: