People told me I'm very good at it, which I find kind of surprising. I'm not sure why that is; it's not like it's something I think of myself as unlikely to be good at. I guess it's probably because it's not a skill that I've worked on consciously, and it seems like something you should work at.
AND I found the heisenbug in the code and managed to scare it away. As in, I was able to reproduce the effect and isolate it to whether a particular block of code was commented-out or not. Then I deleted other code that was commented-out and the bug went away.
At least the plots are done with now.