[R] Three horizontal axes OR Two axes on same side?

[Rafael Jimenez]

Dear list:

I need to reproduce a plot with three different horizontal axes.

I know how to make plot with two different horizontal axes (one
above, one below) using axis():
   axis(1, ....)
   axis(3, ....)

However, I don't know how to produce two axes on the same side of the
plot.

Any pointers or examples?

--
R. Jimenez