text on top just below xticks

Alas, another question.

I want to place text inside the plot, just below the top xaxis ticks.
Previously I had hacked this as:
   sp.text(0.5, 0.96, title_str, font, transform=ax.transAxes)
But as the graphs get taller, the text gets too far away from the top line.

What I think I want is to put the top-aligned text at something like:
(top - markerlen) in pixel coordinates. But where to find the
properties and transforms I need?

Many thanks. I will await a question I can answer.


Charles R. Twardy