Re: [Matplotlib-users] Millions of data points saved to pdf

[Jouni K. S. <jk...@ik...>]

nertskull <ner...@gm...> writes:

> The problem, is the pdf is unbearably slow when plotting as a scatter plot
> or as a line with markers.
>
> If I make a regular line plot, with no markers, just a single line, it is
> plotted and the pdf is fine.  But then it connects my points which I don't
> want.

Others have commented on the volume of data, but that paragraph makes
me curious: are you saying that the results are acceptable if you do
something like

  plot(x, y, '-')

but not if you do

  plot(x, y, 'o')  or  plot(x, y, '-o')?

The amount of data in the pdf file should be within a constant factor in
all cases, but the '-' case there are only moveto and lineto commands,
while the two other cases render markers as something called an XObject,
which is repeated a lot of times on the page. I wonder if the overhead
from using an XObject is making the rendering application slow.

Does it help at all to use a simpler marker, e.g. plot(x, y, ',')? One
change you could try if you're feeling adventurous is the following
function in lib/matplotlib/backends/backend_pdf.py:

    def draw_markers(self, gc, marker_path, marker_trans, path, trans,
                     rgbFace=None):
        # For simple paths or small numbers of markers, don't bother
        # making an XObject
        if len(path) * len(marker_path) <= 10:
            RendererBase.draw_markers(self, gc, marker_path, marker_trans,
                                      path, trans, rgbFace)
            return
        # ...

The comment is not quite right: only if the path is short *and* the
number of markers is small does the XObject code get skipped. You could
just change the if statemt to "if True:" and rerun your code (possibly
with the ',' marker style). If that helps, it's evidence that we need to
revisit the condition for using XObjects for markers.

-- 
Jouni K. Seppänen
http://www.iki.fi/jks