title: Filtering dataframe using Bokeh/Widget/Callback
body: <p>Currently my data set is in the format: <code>Date, Currency, Price</code> which I am filtering at Currency level and then using it to generate graphs.</p>
  
  <p>I want to improve it so that all the filtering is done using Python widget/Dropdown boxes?</p>
  
  <p>I'm new to Python/Bokeh so I need some help.</p>
  
  <pre><code>Date      Currency    Price
  1/1/2017  AUDUSD      1.01
  2/1/2017  AUDUSD      1.02
  3/1/2017  AUDUSD      1.03
  1/1/2017  USDJPY      1.01
  2/1/2017  USDJPY      1.02
  3/1/2017  USDJPY      1.03
  1/1/2017  CADUSD      1.01
  2/1/2017  CADUSD      1.02
  3/1/2017  CADUSD      1.03
  </code></pre>