title: Plotting a contour map with row names/indices of dataframe as y-values
body: <p>The following dataframe:</p>
  <pre><code>import numpy as np
  import pandas as pd
  import matplotlib.pyplot as plt
  
  df = pd.DataFrame(np.random.random((5,5)))
  df.index = [1000, 200, 50, 1, 0.1]
  </code></pre>
  <p>Where the index are pressure levels indicating the altitude of the values.</p>
  <p>When making a simple contour plot:</p>
  <pre><code>plt.contourf(df)
  </code></pre>
  <p><a href="https://i.stack.imgur.com/HOTAl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HOTAl.png" alt="plot" /></a></p>
  <p>The y-values don't correspond with the indices values. I would like to have the pressure levels which correspond to the indices values to be used as the values for the y-axis.</p>
  <p>I tried using the extent parameter, but this stretches out the values between the min/max. Pressure levels aren't spread out evenly.</p>