title: determine matplotlib axis size in pixels
body: <p>Given a set of <a href="http://matplotlib.org/api/axes_api.html" rel="noreferrer">axes</a> in matplotlib, is there a way to determine its size in pixels? I need to scale things according to adjust for larger or smaller figures.</p>
  
  <p>(In particular I want to change the linewidth so it is proportionate for the axes size.)</p>