The feature described in Change Chart Source section of the following article does not seem to work as expected:
When more than one host is selected, with the same target, the drop-down disappears and other hosts cannot be selected - also if more targets are selected for one host, the dropdown disappears as well.
Moreover, when more sources/targets are selected, the same colors of lines are used, which leads to confusion and mess on the chart.
DX UIM 20.4 CU9 / 23.4 CU1 and CU2
Code defect
The engineering team is working on it - this will be fixed in future releases.