John, I think the doc string for points_to_pixels() in

    > backend_bases.py and backend_template.py should be updated
    > define its return type. Does points_to_pixels() return
    > int, float, or float if the backend supports it (agg) else
    > int (gtk)?

I think this should be a float return type, to leave issues of
rounding to the last minute when the backend renderer gets the coords
for drawing. Eg, lines.py uses this information to draw line with
lengths in points - you might imagine a situation where the backend
round one way or the other depending on the two end points of the
line. If the points were rounded first, this would not be possible.
If you agree with this, go ahead and update the doc string.

As I mentioned earlier, I'm looking into refactoring the backend
drawing model so that the transformations happen in the backend, which
would obviate some of these problems.