title: Create Contour Plot from Pandas Groupby Dataframe
body: <p>I have following Pandas Dataframe:</p>
  
  <pre><code>In [66]: hdf.size()
  Out[66]:
  a           b
  0           0.0          21004
              0.1         119903
              0.2         186579
              0.3         417349
              0.4         202723
              0.5         100906
              0.6          56386
              0.7           6080
              0.8           3596
              0.9           2391
              1.0           1963
              1.1           1730
              1.2           1663
              1.3           1614
              1.4           1309
  ...
  186         0.2         15
              0.3          9
              0.4         21
              0.5          4
  187         0.2          3
              0.3         10
              0.4         22
              0.5         10
  188         0.0         11
              0.1         19
              0.2         20
              0.3         13
              0.4          7
              0.5          5
              0.6          1
  Length: 4572, dtype: int64
  </code></pre>
  
  <p>You see, a from 0...188 and b in every group from some value to some value. And as the designated Z-value, the count of the occurence of the pair a/b.</p>
  
  <p>How to get a countour or heatmap plot out of the grouped dataframe?</p>
  
  <p>I have this (asking for the ?):</p>
  
  <pre><code>numcols, numrows = 30, 30
  xi = np.linspace(0, 200, numcols)
  yi = np.linspace(0, 6, numrows)
  xi, yi = np.meshgrid(xi, yi)
  zi = griddata(?, ?, hdf.size().values, xi, yi)
  </code></pre>
  
  <p>How to get the x and y values out of the Groupby object and plot a contour?</p>