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.
I used one of the members of the group to “Analyze” and Generate the Chart. I then saved that chart and from Settings/Pages edited that Chart Page to create what I wanted with the Group included.
From what I could see issue is only with the “Analyze” option off the Group Item.
@Mark_VG Really interesting finding. I tried out your steps and yes, the group item (dark blue line) is displayed in a custom generated chart but not in it’s dedicated analyze chart.