scat2scat: a scatterplot that drives another scatterplot

[kbroman]

Write a function where one clicking on points in one scatterplot generate a second scatterplot.

Input would be a two-column matrix and a list of two-column matrices, where the number of rows in the first matrix is equal to the number of components in the list. Might also allow coloring of groups of points in each of the two scatterplots.

I should probably stick to the style of having each function begin iplot, in which case I'd want to call this iplotScat or iplot2scat, but it seems like scat2scat would be better.

[kbroman]

I implemented this back in May, 2016; see for example this commit.