I have had this happen on my device too with some chargers. Also happened to me with older android phones. Usually happens with cheap chargers/cables/electrical outlets lacking ground leading to AC noise/flunctuation in the DC input signal. lacking a ground wire, Your body becomes the "ground wire" when you touch the screen, sending these flunctuations through the Touchscreen and the you, causing interference. It should get worse if you ground yourself by touching a radiator. Some people reported that they could mitigate the issue by standing on a piece of metal or sitting down on a chair putting their feet up not touching the ground.
Sadly there is no definitve fix for this behavior, you could try a different charger or electrical outlet but that will not necessarily fix it.
Fortunately, allthough annoying, this behavior doesn't cause any lasting damage to the device.
