title: Altair not rendering chart in jupyter notebook
body: <p>I want to view charts in Jupyter notebook using <a href="https://altair-viz.github.io/index.html" rel="nofollow noreferrer">altair</a>. So I tried the <a href="https://github.com/altair-viz/altair_notebooks/tree/master/notebooks" rel="nofollow noreferrer">tutorial examples</a>. I have installed the required packages. 
  This is the code I have:</p>
  
  <pre><code>import altair as alt
  from vega_datasets import data
  
  # Uncomment/run this line to enable Altair in the classic notebook
  #   (this is not necessary in JupyterLab)
  alt.renderers.enable('notebook')
  
  cars = data.cars()
  
  chart = alt.Chart(cars).mark_circle().encode(
      x='Horsepower',
      y='Miles_per_Gallon',
      color='Origin',
  )
  
  chart
  </code></pre>
  
  <p>I am getting a dictionary instead of a chart. Like so:</p>
  
  <pre><code>Chart({
    data:      Acceleration  Cylinders  Displacement  Horsepower  Miles_per_Gallon  \
    0            12.0          8         307.0       130.0              18.0   
    1            11.5          8         350.0       165.0              15.0   
    2            11.0          8         318.0       150.0              18.0   
    3            12.0          8         304.0       150.0              16.0
  </code></pre>
  
  <p>While installing the data sets using this command:</p>
  
  <pre><code>conda install -c conda-forge altair vega_datasets notebook vega3
  </code></pre>
  
  <p>I encountered an error:
  <a href="https://i.stack.imgur.com/1vBcR.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1vBcR.jpg" alt="enter image description here"></a></p>
  
  <p>Could this error be the reason or is there some other problem? </p>
  
  <p>I am using Jupyter lab. With Ipython version > 5. But I am still only getting textual output.
  <a href="https://i.stack.imgur.com/J5kbb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/J5kbb.png" alt="enter image description here"></a></p>