Group difference diff slider endpoint

[akenmorris]

When the group difference slider with the diff option is moved all the way to the right, the difference is zero. This is fine, but the animation is a bit jarring:

vs.

The arrows also point all one direction. The should be 0 length and have disappeared.

The coloring is not a problem as long as the auto scale is not used:

With the auto scale, there is no range, so it's a little wonky

[akenmorris]

Fixed in #2246