title: Python Plotly Multiple Histogram with Mean Line
body: <p>I need to plot two histograms together in Plotly, where each histogram has a line drawn where the mean is, with a label showing the mean value.  My code currently draws both histograms, however I have no idea how to add a mean line with the label.  Any idea?</p>
  
  <pre><code>    import numpy as np
      import random
      from plotly.offline import download_plotlyjs, init_notebook_mode, plot, iplot
      import plotly.graph_objs as go
  
      init_notebook_mode() # run at the start of every ipython notebook
  
      a = np.random.normal(1500, 100, 1000)
      b = np.random.normal(1500, 150, 1000)
  
      trace1 = go.Histogram(
          x=a,
          opacity=0.75,
          histnorm='probability',
          name='&gt; 180 t/h'
      )
      trace2 = go.Histogram(
          x=b,
          opacity=0.75,
          histnorm='probability',
          name='&lt; 160 t/h',
          yaxis='y2'
      )
  
      data = [trace1, trace2]
  
      layout = go.Layout(
          title='title',
          barmode='overlay',
          xaxis=dict(
          title=''
          ),
          yaxis=dict(
              title='Normalized Frequency &lt; 160 t/h'
          ),
          yaxis2=dict(
              title='Normalized Frequency &gt; 180 t/h',
              anchor='free',
              overlaying='y',
              side='right',
              position=1
          )        
      ) 
  
      fig = go.Figure(data=data, layout=layout)
      iplot(fig)
  </code></pre>