Add user preference to control whether discrete / cumulative chart y axis are shared

[dixhuit]

https://www.reddit.com/r/WarhammerCompetitive/comments/1sy3ds6/comment/oiwf0eg/

Currently, there is always 2 y-axis because cumulative always needs a range of 0 to 100% and discrete often needs a much smaller range, this results in the chart bars being taller and easier to read.

Some users would apparently rather have both y-axis use a range of 0 to 100% in which case they may as well share a single y-axis. This is comes down to user preference so let's add a means for the user to toggle this behavior (for all /crunch weapon results charts, not per chart).