Scatterplot by Group on Shared Axes

Scatterplots are a standard data visualization tool that allows you to look at the relationship between two variables \(X\) and \(Y\). If you want to see how the relationship between \(X\) and \(Y\) might be different for Group A as opposed to Group B, then you might want to plot the scatterplot for both groups on the same set of axes, so you can compare them.

Keep in Mind

• Scatterplots may not work well if the data is discrete, or if there are a large number of data points.

Also Consider

• Sometimes, instead of putting both Group A and Group B on the same set of axes, it makes more sense to plot them separately, and put the plots next to each other. See Faceted Graphs.
• There are many ways to make the scatterplots of the two groups distinct. See Styling Scatterplots.

Implementations

R

library(ggplot2)

# Load auto data
data(mtcars)

# Make sure that our grouping variable is a factor
# and labeled properly
mtcars$Transmission <- factor(mtcars$am, 
                              labels = c("Automatic", "Manual"))

# Put wt on the x-axis, mpg on the y-axis, 
ggplot(mtcars, aes(x = wt, y = mpg, 
                   # distinguish the Transmission values by color,
                   color = Transmission)) + 
  # make it a scatterplot with geom_point()
  geom_point()+
  # And label properly
  labs(x = "Car Weight", y = "MPG")

This results in:

Stata

* Load auto data
sysuse auto.dta

* Start a twoway command
* Then, for each group, put its scatter command in ()
* Using if to plot each group separately
* And specifying mcolor or msymbol (etc.) to differentiate them
twoway (scatter weight mpg if foreign == 0, mcolor(black)) (scatter weight mpg if foreign == 1, mcolor(blue))

* Add a legend option so you know what the colors mean
twoway (scatter weight mpg if foreign == 0, mcolor(black)) (scatter weight mpg if foreign == 1, mcolor(blue)), legend(lab(1 Domestic) lab(2 Foreign)) xtitle("Weight") ytitle("MPG")

This results in: