You can find a similar task discussed in this KB article
. It will shed some light on the items ActiveRegion usage, but for the legend there is an additional detail. You need to modify the legend item, and if it is an auto-generated one - it is available in the BeforeLayout
In addition, the items ActiveRegion for line series is only available for the pointmarks, so you will need to set them to be visible. You can still leave their default transparent color.
I have attached a small example.
the Telerik team