title: Plotting only upper/lower triangle of a heatmap
body: <p>In maptplotlib, one can create a heatmap representation of a correlation matrix using the imshow function. By definition, such a matrix is symmetrical around its main diagonal, therefore there is no need to present both the upper and lower triangles. For example:
  <a href="https://i.stack.imgur.com/qx3Tr.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/qx3Tr.jpg" alt="correlation matrix" /></a><br />
  <sub>(source: <a href="https://yin.che.wisc.edu/images/ProteinMatrix.jpg" rel="noreferrer">wisc.edu</a>)</sub></p>
  <p>The above example was taken from <a href="https://katesmaps.blogspot.com/2008/08/correlation-matrix.html" rel="noreferrer">this site</a>
  Unfortunately, I couldn't figure out how to do this in matplotlib. Setting upper/lower part of the matrix to None results in black triangle. I have googled for &quot;matplotlib missing values&quot;, but couldn't find anything helpful</p>