A coordinate singularity at the origin would cause the central pixel
to have a very large value, washing out the color map.
In the process of improving the unit tests, I also corrected the
normalization of the output of the volume renderer. It wasn't essential
for the normalization to be correct because the plot is auto-scaled
anyway. However, it's satisfying that it is correct now anyway.
Fixes https://bugs.ligo.org/redmine/issues/5591.
Original: 7910199784f873777a1a5d484fd6e59dad13bd9c