First, you have to persist the Group state, or there is nothing to chart. This requires manual editing of xxx.persist files at the moment.
Then you have to persuade the UI to offer you a chart for Group type Items, when it would rather tell you about the members.
In sitemap based UIs, this is not even possible and a workaround with proxy Item must be used.
It’s not at all clear to me if MainUI could or should do what you want, but you’re certainly not the only one trying.