title: Adding hover tool tip to bokeh histogram
body: <p>I have created a histogram in <strong>bokeh</strong> using the following code: </p>
  
  <pre><code>TOOLS="pan,wheel_zoom,box_zoom,reset,hover"
  
  for column in valid_columns:
      output_file_name = str( file_name + column + ".html" )
      data_values = stats[ column ].tolist()
  
      output_file( output_file_name )
      histogram, edges = np.histogram( data_values, bins=50 )
  
      source = ColumnDataSource(
          data = dict( data_value = data_values ) )
  
      p1 = figure( title = column, background_fill="#E8DDCB", tools=TOOLS )
      p1.quad( top = histogram, bottom = 0, left = edges[ :-1 ], right = edges[ 1: ], 
               fill_color = "#036564", line_color = "#033649" ) 
  
      hover = p1.select(dict(type=HoverTool))
      hover.tooltips = [ ( "Value", "@data_value" ) ]
  
      show( p1 )
      print( "Saved Figure to ", output_file_name )   
  </code></pre>
  
  <p>where valid columns are a list of all columns I want examined within a <strong>pandas</strong> dataframe. I am trying to add a hover tool tip which will display the number of items stored in each bin but I have not be able to do so. Any help would be appreciated. </p>