title: Asnap hlines and vlines to whole pixels in matplotlib
body: <p>I want to draw some hlines and vlines snapped to occupy whole pixels on the screen,
  not spread across several pixels (rendered, antialiased) as usual.
  Is there a transform T() so that</p>
  
  <pre><code>vlines( T(x), T(ylo), T(yhi), linewidth=Twidth(.5) )
  </code></pre>
  
  <p>draws whole pixels ?  Or, is there a way of telling some Mac backend (I use Qt4agg) to do this ?</p>