Improve GHI shading heatmap

- [ ] Investigate if the first row of points is incorrect (they look very different from the rest of the heat map).
- [ ] Update the mask to exclude flagged points

The issue names no files or tests; start by locating the GHI shading heatmap generation and the mask handling flagged points. Compare the first row of plotted points with the rest and verify that flagged points are excluded. Done means the first-row shading is consistent and the updated mask is reflected in the heatmap.

Written by the indexing model from the issue text.