[[meta title="Looking at matplotlib's cairo backend"]]

[[tag cairo exa xorg]]

The [matplotlib](http://matplotlib.sourceforge.net/) project looks
like an interesting approach for generating charts and things. I used
it (and its cairo backend, plus a couple of patches to snap lines to
pixel boundaries), to generate the following chart:

[[exa/i965/avoiding_rmw/i965.png]]

That chart still isn't perfect as I'd really prefer the spacing
between each of the bars to be consistent, (if you look closely at the
above, you'll notice that some are spaced by 1 pixel and some are
spaced by 2 pixels). Maybe that's a bug in the python script I wrote
to generate the chart, (I'm totally python-clueless).

I sent the pixel-snapping patches up to the matplotlib project, so
maybe they'll go somewhere.

In the meantime, there's been a recent [discussion][1] of how good
matplotlib's cairo backend is compared to some of the others. Here are
some files generated by the unicode_demo example script with
matplotlib 0.87.7, (apparently old, as I see mention of 0.90.1 on the
matplotlib website), and cairo 1.4.10 (or something very close to
it). Note: This is without any of my pixel-snapping patches.

Cairo backend
=============
 * PNG (19023 bytes): [[unicode_demo-cairo.png]]

 * PDF (15786 bytes): [[unicode_demo-cairo.pdf]]

 * PostScript (49279 bytes): [[unicode_demo-cairo.ps]]

 * SVG (89560 bytes): [[unicode_demo-cairo.svg]]

All four of those seem to render fine for me, (with evince, gv, and
rsvg-view, for example). For comparison here are files generated with
other existing matplotlib backends.

AGG backend
===========
 * PNG (19769 bytes): [[unicode_demo-agg.png]]

PDF backend
===========
 * PDF (43539): [[unicode_demo-pdf.pdf]]

PS backend
==========
 * PostScript (136021 bytes): [[unicode_demo-ps.ps]]

SVG backend
===========
 * SVG (7660 bytes): [[unicode_demo-svg.svg]]

Results
=======
All of the cairo results appear to render correctly for me, (using
firefox, evince, gv, and rsvg-view). The PDF generated by the builtin
PDF backend renders with ? characters instead of any of the non-ASCII
glyphs. Also, when rendering the SVG generated by the builtin SVG
backend, the text is not scaled/positioned correctly, (the numbers
overlap the axes, the title at the bottom extends beyond the right
edge instead of being centered, etc.).

Meanwhile, there are a few notable deviations in size from the
backends:

 * Using cairo for PostScript or PDF output results in a file that is
   2.75x smaller than using the builtin PS or PDF backends.

 * Using cairo for SVG output results in a file that is more than 10
   times larger, (it's actually embedding the glyph data for all of the
   text characters that appear in the file).

[1]: http://sourceforge.net/mailarchive/forum.php?thread_name=88e473830707051126uab036cfl2e5bd91bcad72df4%40mail.gmail.com&forum_name=matplotlib-devel