title: Overlaying two histograms with plotly express
body: <p>I'd like to overlay two histograms which I currently display only one next to the other using the following simplistic code. The two dataframes are <em>not the same length</em>, but it still makes sense to overlay their histogram values.</p>
  
  <pre><code>import plotly.express as px
  
  fig1 = px.histogram(test_lengths, x='len', histnorm='probability', nbins=10)
  fig2 = px.histogram(train_lengths, x='len', histnorm='probability', nbins=10)
  fig1.show()
  fig2.show()
  </code></pre>
  
  <p>with pure plotly, this is the way, copied from <a href="https://plot.ly/python/histograms/" rel="noreferrer">the documentation</a>:</p>
  
  <pre><code>import plotly.graph_objects as go
  
  import numpy as np
  
  x0 = np.random.randn(500)
  # Add 1 to shift the mean of the Gaussian distribution
  x1 = np.random.randn(500) + 1
  
  fig = go.Figure()
  fig.add_trace(go.Histogram(x=x0))
  fig.add_trace(go.Histogram(x=x1))
  
  # Overlay both histograms
  fig.update_layout(barmode='overlay')
  # Reduce opacity to see both histograms
  fig.update_traces(opacity=0.75)
  fig.show()
  </code></pre>
  
  <p>I just wonder if there's any particularly idiomatic way with plotly express. Hopefully this also works to exeplify the completeness and different levels of abstraction between plotly and plotly express.</p>