It sounds like you are on the right path to a great visualization. Trend lines are typically over a X axis of Time. Is there a time component to this? Otherwise, it sounds like a scatterplot with Performance scores on one axis and average training score on the other would be the best view to use. The calculation to turn the performance score into a measure could be done quickly as you have suggested and the resulting output would just be the sum of that field.
If there is a date or datetime field that isn't presented here, please share how that is used what the trend line would be indicating (changes in performance over time?)