The size aesthetic is typically used to scale points and text. The default scale for size aesthetics is
scale_size() in which a linear increase in the variable is mapped onto a linear increase in the area (not the radius) of the geom. Scaling as a function of area is a sensible default as human perception of size is more closely mimicked by area scaling than by radius scaling. By default the smallest value in the data (more precisely in the scale limits) is mapped to a size of 1 and the largest is mapped to a size of 6. The
range argument allows you to scale the size of the geoms:
ggplot(mpg, aes(displ, hwy, size = cyl)) + base <- geom_point() base+ scale_size(range = c(1, 2))base
There are situations where area scaling is undesirable, and for such situations the
scale_radius() function is provided. To illustrate when
scale_radius() is appropriate consider a data set containing astronomical data that includes the radius of different planets:
planets#> name type position radius orbit #> 1 Mercury Inner 1 2440 5.79e+07 #> 2 Venus Inner 2 6052 1.08e+08 #> 3 Earth Inner 3 6378 1.50e+08 #> 4 Mars Inner 4 3390 2.28e+08 #> 5 Jupiter Outer 5 71400 7.78e+08 #> 6 Saturn Outer 6 60330 1.43e+09 #> 7 Uranus Outer 7 25559 2.87e+09 #> 8 Neptune Outer 8 24764 4.50e+09
In this instance a plot that uses the size aesthetic to represent the radius of the planets should use
scale_radius() rather than the default
scale_size(). It is also important in this case to set the scale limits so that a planet with radius 0 would be drawn with a disc with radius 0.
ggplot(planets, aes(1, name, size = radius)) + base <- geom_point() + scale_x_continuous(breaks = NULL) + labs(x = NULL, y = NULL, size = NULL) + ggtitle("not to scale") base + base scale_radius(limits = c(0, NA), range = c(0, 10)) + ggtitle("to scale")
On the left it is difficult to distinguish Jupiter from Saturn, despite the fact that the difference between the two should be double the size of Earth; compare this to the plot on the right where the radius of Jupiter is visibly larger.
Other size scales exist and are worth noting briefly:
scale_size_binned()is a size scale that behaves like
scale_size()but maps continuous values onto discrete size categories (analogous to the binned position and colour scales discussed earlier)
scale_size_binned_area()are versions of
scale_size_binned()that ensure that a value of 0 maps to an area of 0.
scale_size_datetime()are designed to handle date data, analogous to the date scales discussed earlier.