Ah, how frustrating! After making the pen and submitting this post, I then explored using the update method which had overwritten my original pen...which didn't work.
The solution you proposed with extendTraces was exactly how I had it before, but when you click the start button, the new values are missing the z value at y0?
I had this problem too. Is it possible to make extendTraces add in values like you did with the update function?
Thank you so much for your reply!
edit: It's missing the z value at b0,y0