title: Make output pdf files binary identical
body: <p>I'm using Matplotlib to produce a large batch of plots (order of thousands). I'll frequently tweak minor things in the code that produces the plots, but the changes will often only affect a few plots. </p>
  
  <p>When I push new plots to a common repository, I'd like to use something like <code>rsync</code> or <code>diff</code> to identify which plots have actually changed. Unfortunately, running <code>diff new_plot.pdf old_plot.pdf</code> <em>always</em> identifies the files as different, even if <em>nothing</em> about the plotting script has changed. </p>
  
  <p>When I output to <code>.png</code> the files are identical. When I output to an <code>.eps</code> the output is nearly identical, but running diff shows that a few lines have been swapped. I suspect there are two reasons for the difference:</p>
  
  <ol>
  <li>A pdf stores metadata, including a time-stamp. </li>
  <li>Some vector graphics may look identical even without the same description (i.e. a line could be drawn from right to left or from left to right). I'd expect matplotlib to be deterministic, but it clearly does things slightly differently in the <code>.eps</code> instance, so I guess not.</li>
  </ol>
  
  <p>Is there some way to turn off <code>.pdf</code> metadata, and either force a more deterministic drawing method out of matplotlib or feed the files to a diff tool that sees them as identical?</p>