title: Plot multiple Y axes
body: <p>I know pandas supports a secondary Y axis, but I'm curious if anyone knows a way to put a tertiary Y axis on plots. Currently I am achieving this with numpy+pyplot, but it is slow with large data sets.</p>
  <p>This is to plot different measurements with distinct units on the same graph for easy comparison (eg: Relative Humidity/Temperature/ and Electrical Conductivity).</p>
  <p>So really just curious if anyone knows if this is possible in <code>pandas</code> without too much work.</p>
  <p>[Edit] I doubt that there is a way to do this(without too much overhead) however I hope to be proven wrong, as this may be a limitation of matplotlib.</p>