Why is the screen acting weird while charging? I noticed that when I charge my phone with a cheap charger that I bought from Ebay, the screen acts strange:


It suddenly can't position my taps correctly and
It detects swipes as taps. 


After a fast googling the reason is most likely that the charger has a bigger voltage output that the phone expects and that causes the trouble. 

Why does a bigger voltage output cause a behaviour like this?
Problems with ghost touches on the screen while charging is very common when using third party chargers or cables. It is caused by electrical discharge in the form of either current or static charge. I'd toss that charger and/or cable away and buy an original one. Damage might have been cause already though which will most likely be seen by unusual battery draining (it can mess up the  calibration).