Used (probably) inappropriate charger, touchscreen went crazy, permanent damage?
You can be 99.9% sure the touchscreen is fine. Battery wasn't damaged too. If the touchscreen still behaves erroneously after unplugging, then rubbing it with a soft cloth should help. 

This is a common issue with cheap chargers. It indicates that charger may not be safe to use with any device - it may catch fire or explode. Prolonged use of such chargers can degrade battery performance faster than charging with a high-quality charger. 

It wasn't a voltage problem - all USB chargers have the same voltage and you can use any charger with a plug that fits. Charging speed may differ, though - high-end smartphone and tablet chargers are faster than regular mobile phone ones.