title: I am not able to display graph using python datashader
body: <p>I downloaded and installed datasheder using the below steps:</p>
  
  <pre><code>git clone https://github.com/bokeh/datashader.git  
  cd datashader  
  conda install -c bokeh --file requirements.txt  
  python setup.py install
  </code></pre>
  
  <p>After that, I have run the code using terminal like `python data.py, but no graph is displayed; nothin is being displayed.</p>
  
  <p>I am not sure if I've follwed the right steps here, can somebody help me display the graphs? Here is my code:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  import xarray as xr
  import datashader as ds
  import datashader.glyphs
  import datashader.transfer_functions as tf
  from collections import OrderedDict
  
  np.random.seed(1)
  num=10000
  
  dists = {cat: pd.DataFrame(dict(x=np.random.normal(x,s,num),
                              y=np.random.normal(y,s,num),
                              val=val,cat=cat))
       for x,y,s,val,cat in 
       [(2,2,0.01,10,"d1"), (2,-2,0.1,20,"d2"), (-2,-2,0.5,30,"d3"), (-2,2,1.0,40,"d4"), (0,0,3,50,"d5")]}
  
  df = pd.concat(dists,ignore_index=True)
  df["cat"]=df["cat"].astype("category")
  df.tail()
  
  tf.shade(ds.Canvas().points(df,'x','y'))
  glyph = ds.glyphs.Point('x', 'y')
  canvas = ds.Canvas(plot_width=200, plot_height=200, x_range=(-8,8)y_range=(-8,8))
  from datashader import reductions
  reduction = reductions.count()
  
  from datashader.core import bypixel
  agg = bypixel(df, canvas, glyph, reduction)
  agg
  canvas.points(df, 'x', 'y', agg=reductions.count())
  tf.shade(canvas.points(df,'x','y',agg=reductions.count()))
  tf.shade(canvas.points(df,'x','y',agg=reductions.any()))
  tf.shade(canvas.points(df,'x','y',agg=reductions.mean('y')))
  tf.shade(50-canvas.points(df,'x','y',agg=reductions.mean('val')))
  agg  = canvas.points(df, 'x', 'y')
  tf.shade(agg.where(agg&gt;=np.percentile(agg,99)))
  tf.shade(np.sin(agg))
  aggc = canvas.points(df, 'x', 'y', ds.count_cat('cat'))
  aggc
  tf.shade(aggc.sel(cat='d3'))
  agg_d3_d5=aggc.sel(cat=['d3', 'd5']).sum(dim='cat')
  tf.shade(agg_d3_d5)
  </code></pre>