title: Single axis caption in plotly express facet plot
body: <p>I am learning to use pyplot.express and struggle with the following design problem: In faceted plots, the axis title is repeated for every subplot (in the example case 'petal width (cm)'). Is there a way to get a single axis label for all subplots on faceted plots using pyplot.express?</p>
  
  <p>thanks, Michael</p>
  
  <p>Minimal example:</p>
  
  <pre><code>from sklearn.datasets import load_iris
  import plotly.express as px
  import pandas as pd
  import numpy as np
  
  # import iris-data
  iris = load_iris()
  df= pd.DataFrame(data= np.c_[iris['data'], iris['target']], columns= iris['feature_names'] + ['target'])
  df['species'] = pd.Categorical.from_codes(iris.target, iris.target_names)
  
  # plot using pyplot.express
  fig = px.bar(df, x="sepal length (cm)", y="petal width (cm)", color = 'petal length (cm)', facet_row="species")
  fig.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ZkdaB.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZkdaB.png" alt="Iris facet plot"></a></p>