Some people have suggested adding a feature to draw lines by long pressing a square, but I think that could create conflicts with the piece drag and drop since you could be just trying to drag a piece, so instead I suggest adding a button, that allows to draw lines WHILE it is PRESSED. So the idea is that you use 2 fingers to draw, one finger is pressing the button and the other is actually drawing, so that if you wanted to stop drawing to quickly play a move you could just release the button and make the move