show2D stretches datasets differently based on number of angles

[lauramurgatroyd]

Description

There should be an option to prevent stretching horizontally based on number of angles.

An example of what occurs. Each dataset title shows the number of angles in the data.
All datasets have the same size on the horizontal axis

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)