sum component vectors are not visible in some cases

[pixelzoom]

Discovered while working on #225.

This bug that has been around for awhile. It slipped through dev testing in 1.0.0-dev.36, and it's still present in master.

1. Start sim, go to Explore 2D screen.
2. Check the "Sum" checkbox.
3. Choose any of the Components radio buttons except "invisible"
4. Drag 'a' from toolbox to graph
5. Components will be shown for 'a', but not for the sum. See screenshot below.

Moving 's', changing the Components setting, or toggling the "Sum" checkbox will show the sum component vectors.

[pixelzoom]

Fixed in master and 1.0.0-dev.41. Closing.

[arouinfar]

I ran into this while reviewing interview footage for #230. I was unable to reproduce in dev.41. Thanks @pixelzoom!