Teams user activity report does not include data to reproduce graph
This is a Teams Admin center issue.
When viewing the Analytics & reports | Usage reports, you get a graph and a table.
But if you use the Export to Excel button, it only exports the table data and not the data used to create the graph. So you can't re-create the graph.
Ideally we should be able to get access to the same kinds of reports that are available in on-prem (and cloud?) Skype for Business, but these just seem to be lacking for Teams. So I'd settle for just being able to reproduce the graph that's on the Teams user activity report page.
There is similar data available in the O365 Admin center | Reports | Usage | Microsoft Teams user activity report - but it doesn't have the same columns available.
There is similar data available in the Power BI Microsoft 365 Usage Analytics app | Product Usage | Teams - but it doesn't have the same columns and it summarizes to the month, not day.