title: Pandas plot multiple category lines
body: <p>Say I have the following data...</p>
  
  <pre><code>date      Score  category                 
  2017-01-01   50.0         1
  2017-01-01  590.0         2
  2017-01-02   30.0         1
  2017-01-02  210.4         2
  2017-01-03   11.0         1
  2017-01-03   50.3         2
  </code></pre>
  
  <p>So on a daily basis, I have multiple categories, each being assigned a score. 
  Here's my code so far...</p>
  
  <pre><code>vals = [{'date': '2017-01-01', 'category': 1, 'Score': 50},
           {'date': '2017-01-01', 'category': 2, 'Score': 590},
           {'date': '2017-01-02', 'category': 1, 'Score': 30},
           {'date': '2017-01-02', 'category': 2, 'Score': 210.4},
           {'date': '2017-01-03', 'category': 1, 'Score': 11},
           {'date': '2017-01-03', 'category': 2, 'Score': 50.3}]
  df = pd.DataFrame(vals)
  df.date = pd.to_datetime(df['date'], format='%Y-%m-%d')
  df.set_index(['date'],inplace=True)
  </code></pre>
  
  <p>Which results in a bizarre plot as below.
  <a href="https://i.stack.imgur.com/hXhsf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hXhsf.png" alt="enter image description here"></a></p>
  
  <p>I'd like to have multiple lines, one for each category, and the date on the X-axis - how would I do this?</p>