As a general solution for passing further args
to imagemagick when generating animations,
extra_args is now actually used. Extra args are
placed after all other args, but before the
output file name.

PR Summary

Note: This is a "general solution" to the problem I had.

My usecase: I found that if I passed a generated .gif back to Imagemagick, with the args -layers Optimize (more fully just: convert anim.gif -layers Optimize anim.gif), My several-megabyte gifs would be reduced in size by ~10x. In my current usage, I do this second pass via subprocess.

I then went ahead and modified my local copy of matplotlib (caveat: v1.5.1 with pyhon2.7 on ubuntu 14.04; but the code looks identical between master and 1.51).

With the fix in this PR, I can now do anim.save(gif_path, dpi=80, writer='imagemagick', extra_args=["-layers", "Optimize"]). This works fine giving visually identical animations, and further reduces file size another order of magnitude (in the one case I'm dealing with, I'm getting a resulting file 1/70th the size).

A concern I have (as I am not a regular imagemagick user, let alone an expert) is whether simply dumping extra args to image magick at the end of the hardcoded args is reasonable, or if order of operations is likely to make a mess with certain additional imagemagick arguments/operations... I might argue that any user adding imagemagick args probably knows what they're doing and would realize what's up. (I definitely benefited from matplotlib.debug=helpful or something like that, which showed the convert command that was actually invoked.) Alternately, maybe this is a spot where it's desirable to have more detailed documentation?

For reference, the code where convert is invoked with args looks like:

return ([self.bin_path(),                                  
         '-size', '%ix%i' % self.frame_size, '-depth', '8',
         '-delay', str(self.delay), '-loop', '0',          
         '%s:-' % self.frame_format]                       
        + self.output_args)                                

Therefore I am completely open to revising/redoing this PR and instead adding a ImageMagick[File]Writer specific arg/property e.g. compress that explicitly adds -layers Optimize to the command (and leave extra_args unused as it currently is).

Other notes:

• I chose to modify the output_args property with additional logic, as this is similar to what is done with FFMpegBase and MencoderBase classes
  • Alternately, I probably would have just modified the _args() function instead.
• I verified that both ImageMagickWriter and ImageMagickFileWriter classes use the extra args as expected

PR Checklist

• Has Pytest style unit tests no similar tests exist for other classes using extra_args
• Code is Flake 8 compliant no additional warnings generated
• New features are documented, with examples if plot related
• [N/A] Documentation is sphinx and numpydoc compliant
• [N/A] Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• [N/A] Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way