title: Plotly-Express: How to fix the color mapping when setting color by column name
body: <p>I am using <code>plotly express</code> for a scatter plot. The color of the markers is defined by a variable of my dataframe, as in the example below.</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  import plotly.express as px
  
  df = px.data.iris()
  
  fig = px.scatter(df[df.species.isin(['virginica', 'setosa'])], x="sepal_width", y="sepal_length", color="species")
  fig.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/9I1MK.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9I1MK.png" alt="enter image description here"></a></p>
  
  <p>When I add another instance of this variable, the color mapping changes (First, 'virginica', is red, then green).</p>
  
  <pre><code>fig = px.scatter(df, x="sepal_width", y="sepal_length", color="species",size='petal_length', hover_data=['petal_width'])
  fig.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/y7x2P.png" rel="noreferrer"><img src="https://i.stack.imgur.com/y7x2P.png" alt="enter image description here"></a></p>
  
  <p>How can I keep the mapping of the colors when adding variables?</p>