ggplot with 2 y axes on each side and different scales
I need to plot a bar chart showing counts and a line chart showing rate all in one chart, I can do both of them separately, but when I put them together, I scale of the first layer (i.e. the geom_bar) is overlapped by the second layer (i.e. the geom_line).
Can I move the axis of the geom_line to the right?
Answer
It's not possible in ggplot2 because I believe plots with separate y scales (not y-scales that are transformations of each other) are fundamentally flawed. Some problems:
The are not invertible: given a point on the plot space, you can not uniquely map it back to a point in the data space.
They are relatively hard to read correctly compared to other options. See A Study on Dual-Scale Data Charts by Petra Isenberg, Anastasia Bezerianos, Pierre Dragicevic, and Jean-Daniel Fekete for details.
They are easily manipulated to mislead: there is no unique way to specify the relative scales of the axes, leaving them open to manipulation. Two examples from the Junkcharts blog: one, two
They are arbitrary: why have only 2 scales, not 3, 4 or ten?
You also might want to read Stephen Few's lengthy discussion on the topic Dual-Scaled Axes in Graphs Are They Ever the Best Solution?.