Boxplot Group By filters cannot be reenabled

[dchiquito]

Steps to reproduce:

1. Load some data with multiple groups defined (I used Cancer chemo liver (NMR) from the samples)
2. Go to Analyze Table > Boxplots
3. Under Group By, change No grouping to Treatment
4. You should see bars of all colors defined for all groups
5. Deselect a group
6. You should see that groups color disappear from the boxplot, as expected
7. Select that group again
8. The boxplot rerenders and shifts to make room for the group, but does not render it.

Toggling various groups on/off in various combinations can produce similar errors where groups are not rendered.

Possibly incidentally, changing Group By from Treatment to No grouping works, but deletes the text from the Group By box.