label disappears on sum vector

[pixelzoom]

Steps to reproduce:

1. start sim, go to Explore 2D screen
2. drag vector 'a' onto graph
3. check 'Sum' checkbox, note that sum vector appears, labeled with 's'
4. drag sum vector 's'
5. uncheck 'Sum' checkbox
6. check 'Sum' checkbox, note that sum vector appears, but without label 's' (screenshot below)
7. select the sum vector and the label 's' will reappear

[pixelzoom]

Very odd, introduced in 96eb7e7. Fixed in the above commit.

Closing.