title: Overlay two Data Frames in one graph
body: <p>I have two different dataframes both are the same size and have the same values for the Date column.</p>
  
  <pre><code>        Date     Open     High      Low    Close        Volume    Market Cap
  0 2018-04-16  8337.57  8371.15  7925.73  8058.67  5.631310e+09  1.415710e+11
  1 2018-04-15  7999.33  8338.42  7999.33  8329.11  5.244480e+09  1.358120e+11
  2 2018-04-14  7874.67  8140.71  7846.00  7986.24  5.191430e+09  1.336820e+11
  3 2018-04-13  7901.09  8183.96  7758.93  7895.96  7.764460e+09  1.341140e+11
  4 2018-04-12  6955.38  7899.23  6806.51  7889.25  8.906250e+09  1.180480e+11
  </code></pre>
  
  <p>Both data frames have the same structure. same column indexes and same number of rows.</p>
  
  <pre><code>print(df.dtypes)
  print(df2.dtypes)
  </code></pre>
  
  <p>Both give</p>
  
  <pre><code>Date          datetime64[ns]
  Open                 float64
  High                 float64
  Low                  float64
  Close                float64  
  Volume               float64
  Market Cap           float64
  dtype: object
  </code></pre>
  
  <p>I would like to plot Date on the x axis and Market Cap on the y axis. From my research I believe it is not possible to merge the plot function of a dataframe ie.</p>
  
  <pre><code>df.plot(x='Date', y = 'Market Cap', kind = 'line')
  df2.plot(x='Date', y = 'Market Cap', kind = 'line')
  </code></pre>
  
  <p>My question is what is a good method to obtain the graph that I am trying to create? Should I use groupby, keys, or pyplot</p>
  
  <p>to be clear i would like an overlay of the separate two graphs:</p>
  
  <p><img src="https://i.stack.imgur.com/QI15T.png" alt="seperate two graphs"></p>
  
  <p>So far I have tried the code:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  
  # Init subplots
  fig, axes = plt.subplots(1,1);
  
  # Init ax with the first plot.
  ax = df['Market Cap'].plot()
  
  # Plot second df using ax from the first plot.
  _ = df2['Market Cap'].plot(ax = ax)
  
  plt.show()
  </code></pre>
  
  <p>Which results in an improper overlay:</p>
  
  <p><img src="https://i.stack.imgur.com/X8rNcm.png" alt="results in improper overlay"></p>