In the attached file I point out a missing feature.
At first I want to mirror-copy the line "A" specifying axis "B" as reference to get line "A1".
Grouping the elements marked with "C", I want to mirror-copy on axis "D" to get an object "C1".
The final result is marked "E".
Unfortunately, there is no option to specify a reference point for mirroring in the Transform docker like in „Rotate: Specify Point“.
You have to poke around with "helper" objects to specify an anchor point to get the desired mirror-copy object, but you have to delete these helper objects afterwards. This is awkward.