title: plotly basic example shows no plot in jupyter lab
body: <p>I just tried to use plotly offline in a jupyter notebook (using jupyter lab).  The <a href="https://plot.ly/python/getting-started/#start-plotting-online" rel="noreferrer">docs</a> gives this example:</p>
  
  <pre><code>import plotly
  import plotly.graph_objs as go
  
  plotly.offline.init_notebook_mode(connected=True)
  
  plotly.offline.iplot({
      "data": [go.Scatter(x=[1, 2, 3, 4], y=[4, 3, 2, 1])],
      "layout": go.Layout(title="hello world")
  })
  </code></pre>
  
  <p>I tried it and there is no chart displayed, either in Edge or Firefox.<br>
  An output cell is made in the notebook, but there is no chart.</p>
  
  <p>It works if I use plotly.offline.plot rather than iplot.  It works if I use the traditional jupyter notebook rather than jupyter lab.</p>
  
  <p>Any ideas why it does not display?</p>
  
  <p>It's plotly version 2.7.0.</p>