title: Split up time series per year for plotting
body: <p>I would like to plot a time series, start Oct-2015 and end Feb-2018, in one graph, each year is a single line. The time series is <code>int64</code> value and is in a <code>Pandas DataFrame</code>. The date is in <code>datetime64[ns]</code> as one of the columns in the DataFrame.</p>
  
  <p>How would I create a graph from Jan-Dez with 4 lines for each year.</p>
  
  <p>graph['share_price'] and graph['date'] are used. I have tried <code>Grouper</code>, but that somehow takes Oct-2015 values and mixes it with the January values from all other years.</p>
  
  <p>This groupby is close to what I want, but I loose the information which year the index of the list belongs to.</p>
  
  <pre><code>graph.groupby('date').agg({'share_price':lambda x: list(x)})
  </code></pre>
  
  <p>Then I have created a DataFrame with 4 columns, 1 for each year but still, I don't know how to go ahead and group these 4 columns in a way, that I will be able to plot a graph in a way I want.</p>