[R] split a y-axis to show data on different scales

[hadley wickham]

> How do I split a y-axis to plot data on different scales?

The short answer: you shouldn't.  The whole point of plotting the data
is so that you can compare them visually on the same scale.  As soon
as you split the scales you can no longer do this, and you effectively
have two separate graphs.  This suggests how you should solve your
problem - create two graphs, one for each separate scale.

Hadley