title: how do use a Python colormap in two directions
body: <p>I have some data that I am trying to plot, that goes from -1 to 0 to 1 and I would like to use a color bar such as &quot;Viridis&quot; but I would like to have the same colors run from -1 (yellow) to 0 (purple), and then flip directions and from from 0 (purple) to 1 (yellow). Essentially have the purple in the middle at 0 and have the exact same transitions in colors for 0 to -1 and 0 to 1. In this instance I could just have the values has aboslute values but moving forward with the code, I wont be able to do that for a variety of reasons.</p>
  <p>any help is appreciated!</p>