title: plotly graph edge label
body: <p>I want to add label to Graph in <code>plotly</code>. I wrote a program like this but it doesn't work</p>
  
  <pre><code>edge_trace = Scatter(
      x=[],
      y=[],
      line=Line(width=3, color='rgba(136, 136, 136, .8)', text=[]),
      hoverinfo='none',
      text='',
      mode='lines')
  
  for edge in G.edges():
      x0, y0 = pos[edge[0]]
      x1, y1 = pos[edge[1]]
      edge_trace['x'] += [x0, x1, None]
      edge_trace['y'] += [y0, y1, None]
      edge_trace['line']['text'].append('label')
  </code></pre>
  
  <p>I got the following error:</p>
  
  <pre><code>    raise exceptions.PlotlyDictKeyError(self, path)
      plotly.exceptions.PlotlyDictKeyError: 'line' is not allowed in 'line'
  </code></pre>
  
  <p>What can I do?</p>