title: Histogram with slider filter
body: <p>I would like to create a histogram with a density plot combined in bokeh with a slider filter. Atm, I have the blocks to create a bokeh histogram with a density plot from another thread. I dont know how to create the callback function to update the data and rerender the plot. </p>
  
  <pre><code>from bokeh.io import output_file, show
  from bokeh.plotting import figure
  from bokeh.sampledata.autompg import autompg as df
  
  from numpy import histogram, linspace
  from scipy.stats.kde import gaussian_kde
  
  pdf = gaussian_kde(df.hp)
  
  x = linspace(0,250,50)
  
  p = figure(plot_height=300)
  p.line(x, pdf(x))
  
  # plot actual hist for comparison
  hist, edges = histogram(df.hp, density=True, bins=20)
  p.quad(top=hist, bottom=0, left=edges[:-1], right=edges[1:], alpha=0.4)
  
  show(p)
  </code></pre>