tick labels plot_date

I have about 20 years of monthly data to plot
using plot_date. The data are datetime dates
and floats. As usual, the ticks are chosen
very nicely for the floats. But far too many dates
are ticked, and their text completely overlaps.
Even if I use autofmt_xdate(), which allows me
to at least read them, they are far too crowded.

What is the simplest way to force fewer
date labels?

Alan Isaac