Can you show me the full expression that you are currently using in the expression box (the lower image, in the properties panel)? Likely just a syntax error. Also, the image on the right (Means) graph looks skewed as well, correct?
As far as I can tell, the expression in the stacked bar chart in question has a set analysis statement that's referencing a field name 'RESPONSE_TEXT' that I do not see in the screenshot of the tables. Is 'RESPONSE_TEXT' a field that is present in the data model you sent me a screenshot of and I cannot see it? If not, what field corresponds to the 'Strongly Disagree' to 'Strongly Agree' values?
Also, once you've determined the field that corresponds with the 'Strongly Disagree' - 'Strongly Agree' values, try setting up your graph as such:
In this example I've set up a chart where I used a stacked bar chart to show me the number of sales each of my sales team made each year.
I gave each set analysis expression it's own measure. In my example, I wanted to show yearly sales by Salesperson, so I used a set analysis statement for each year and gave it a separate measure. Similarly, you want to use a set analysis expression for the student response and give each it's own measure.
See if this helps. I think there may be some data model issues, as you stated. Again, first find the field that corresponds to the values you are looking for, then generate a separate measure using a set analysis statement for each, similarly to how I did above.
My apologies, I see you wanted the graph set up to represent portion of the whole. I will revisit my response. In the meantime, please reference the other part of my response regarding data model and field name.