Often user choose a multi select field and then run into all kinds of data analysis issues as they do not really lend themselves to any kind of reporting. The Pie charts would be done off the child records, and this new feature coming out on December 16th will allow the unique child values to be summarized up to the parent as a text string.
I had the opportunity to see this is action on a test app as I was giving feedback to Product Manager. This will save me a lot of time in explaining how to do reverse relationships, here on this forum, to float up text values from children to parents, as now it will be a native feature.