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In their most basic form scatter plots can only visualize datasets in two dimensions through the x and y aesthetics of the geom_point() layer. However, most data sets have more than two variables and thus might require additional plotting dimensions. ggplot() makes it very easy to map additional variables to different plotting aesthetics like size, transparency alpha and color.
Let’s consider the gapminder_2007 dataset which contains the variables GDP per capita gdpPercap and life expectancy lifeExp for 142 countries in the year 2007:
ggplot(gapminder_2007) +
geom_point(aes(x = gdpPercap, y = lifeExp))
Mapping the continent variable through the point color aesthetic and the population pop (in millions) through the point size we obtain a much richer plot including 4 different variables from the data set: