How to create an average line over 2 dimensions?

[#1]

Hi,

I am trying to create an average score per QuestionType per student/ resident.  However, whenever I put in one more expression (the average score), the chart legend changes which is not what I want. 

Below are the screenshot to explain:

The below chart is the chart I need to show, only lacking is an average of all the question types grouped by per resident.

Second screenshot shows that my QuestionType changed to score when I put in average line expression. 

How should i put in my average line so that the chart will remain as if shown in screenshot 1.

thanks!