Hi Boofhead,
I am not a pilot, so I am having difficulty grasping what your chart is trying to show.
However, I do know a bit about excel. I think your problem is the 'choose' function. Excel has several different data types: eg numeric, text.
My suspicion is that the choose function is returning a string or text data type, so that when you try to chart it it interprets it as a Null value - hence the line disappears.
What the "=C31+0" is doing is forcing it into numeric data type, and so it is possible to chart the result.
HTH
EG