Align text using advance width, not glyph width

Hello, all. I've noticed that the alignment of text strings is usually
based on the bounding box of the composite glyphs--the painted region. In
certain cases, this behavior is producing results that I think are
undesirable. One example involves the common case of fonts with tabular
figures (numerals), for which the advance widths are equal (so that the
digits line up in columnar layouts like spreadsheets), but whose glyphs have
different widths (such as a digit one that's narrower than the other
digits). Using such a font, when a Y axis label ends with a one,
matplotlib's glyph-based right alignment takes that labels' digits out of
vertical alignment with the other labels. Similar effects occur with
old-style figures on the X axis, although that's more rarely seen in
practice, I would imagine.

More details and an example script and image are in the support request
706&atid=560721, although I'm not sure whether this issue constitutes a
support request, a feature request, or something else.

What would be required to align text horizontally using advance widths and
vertically using font-wide metrics for ascent, descent, and so on? Thanks.