CX dashboard simple chart percentage of respondents

I tried to get help with this problem through Qualtrics support and they suggested I post here. The survey question is a multi response multiple choice where respondents selected all choices that applied to them. I want a simple chart, line chart of the percentages of respondents in each group (cohort) for each year of the fellowship (3, 5, 6) out of the total number of respondents for that cohort/fellowship year that took the survey. I can get the counts but when I try to do the percentages, it's dividing by the count instead of the total respondents (even when I choose count/total column). For example, there were 16 people from the 2018 cohort in Year 3 of the fellowship who selected choice A out of 26 total respondents but when I chose count/total column, the percentage shows as 100% (16/16) instead of 62% (16/26).
I sent an explanation to Qualtrics support and their response was: "Qualtrics line charts are automatically set up to only convey completed responses instead of a percentage of completed responses out of the total possible responses. You will need to create your own custom metric in order for the line chart to reflect your desired result." They referred me to the custom metrics support page.
I couldn't figure out the equation for the custom metric that would give me what I want and, when I asked for help, was advised by Qualtrics that "It's standard procedure to direct you to instructions and help you troubleshoot, but anything that needs to be customized is out of our scope. I sincerely apologize for this." They suggested I post the issue here. Thanks in advance for any help you can give.