How can I plot with 2 different y-axes?

Question

I would like superimpose two scatter plots in R so that each set of points has its own (different) y-axis (i.e., in positions 2 and 4 on the figure) but the points appear su

Answer 1

As its name suggests, twoord.plot() in the plotrix package plots with two ordinate axes.

library(plotrix)
example(twoord.plot)