1 Goal

The goal of this tutorial is to learn different ways to introduce a third variable in a scatter plot in ggplot using shape, colour and size in geom_point.

2 Drawing with ggplot

# First we load the libraries
library(ggplot2)

# In this example we will use the open repository of plants classification Iris. 
data("iris")
str(iris)
## 'data.frame':    150 obs. of  5 variables:
##  $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
##  $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
##  $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
##  $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
##  $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1 1 1 1 1 ...
# Let's plot the petal length vs petal width
ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length))

3 Adding a third variable in geom point

3.1 Using colour

# We can add a third variable to the plot using colour
# Notice that the color parameter should be set inside of the aes function, otherwise the effect will be different 
ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length, color = Species)) 

# See what happens if we define de colour outside
# ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length), color = iris$Species) 
# Error in grDevices::col2rgb(colour, TRUE) : invalid color name 'setosa'

# ggplot is expecting a name of a colour and insted it gets the character "setosa", then it doesn't know what to do
# However we could use a defined colour and it will work

ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length), color = "blue")

3.2 Changing the shape of the data points

# ggplot comes with predefined shapes for the data points
# We can ask different shapes for different values of the third variable
ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length, shape = Species)) 

# Notice that we could combine both parameters in order to separate better points that overlap
ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length, shape = Species, color = Species))

3.3 Changing the size of the data points

# We can define the size of the data points as a variable
# Using size for a discrete variable is not advised.

ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length, size = Species))

# Again it can be combined with the other parameters for better separation of the third variable points
ggplot() + geom_point(data = iris, aes(x = Petal.Width, y = Petal.Length, size = Species, color = Species, shape = Species))

4 Conclusion

In this tutorial we have learnt how to use different parameters as shape, colour and size to introduce a third variable in our scatter plots using geom_point.