Python Plotly Multiple Histogram with Mean Line
<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>