title: Plotly dash ignore NaN values
body: <p>I have some NaN values in my dataframe like this:</p>
  
  <pre><code>a|b|c|d
  a1|NaN|c1|cd
  a2|b2|c1|NaN
  </code></pre>
  
  <p>When I draw a parallel categories diagram, they are reflecting in the plot.How do I make them invisible in parallel categories diagram?
  I use the following code:</p>
  
  <pre><code>fig = px.parallel_categories(df)
  
  fig.show()
  </code></pre>