title: How to plot vertical lines in plotly offline?
body: <p>How would one plot a vertical line in plotly offline, using python? I want to add lines at x=20, x=40, and x=60, all in the same plot.   </p>
  
  <pre><code>def graph_contracts(self):
      trace1 = go.Scatter(
          x=np.array(range(len(all_prices))),
          y=np.array(all_prices), mode='markers', marker=dict(size=10, color='rgba(152, 0, 0, .8)'))
      data = [trace1]
      layout = go.Layout(title='Market Contracts by Period',
                         xaxis=dict(title='Contract #',
                                    titlefont=dict(family='Courier New, monospace', size=18, color='#7f7f7f')),
                         yaxis=dict(title='Prices ($)',
                                    titlefont=dict(family='Courier New, monospace', size=18, color='#7f7f7f')))
      fig = go.Figure(data=data, layout=layout)
      py.offline.plot(fig)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/iXhBq.png" rel="noreferrer"><img src="https://i.stack.imgur.com/iXhBq.png" alt="Generated graph"></a></p>