title: Color points in scatter plot of Bokeh
body: <p>I have the following simple <code>pandas.DataFrame</code>:</p>
  
  <pre><code>df = pd.DataFrame(
      {
          "journey": ['ch1', 'ch2', 'ch2', 'ch1'],
          "cat": ['a', 'b', 'a', 'c'],
          "kpi1": [1,2,3,4],
          "kpi2": [4,3,2,1]
      }
  )
  </code></pre>
  
  <p>Which I plot as follows:</p>
  
  <pre><code>import bokeh.plotting as bpl
  import bokeh.models as bmo
  bpl.output_notebook()
  source = bpl.ColumnDataSource.from_df(df)
  hover = bmo.HoverTool(
      tooltips=[
          ("index", "@index"),
          ('journey', '@journey'),
          ("Cat", '@cat')
      ]
  )
  p = bpl.figure(tools=[hover])
  
  p.scatter(
      'kpi1', 
      'kpi2', source=source)
  
  bpl.show(p)  # open a browser
  </code></pre>
  
  <p>I am failing to color code the dots according to the <code>cat</code>. Ultimately, I want to have the first and third point in the same color, and the second and fourth in two more different colors.</p>
  
  <p>How can I achieve this using Bokeh?</p>