Joannesalfa : I didn't played Mage Gauntlet on iOS, but it's known for that kind of mechanism (perhaps you played it yourself, and it's given you the idea ? <img src="smileys/smiley2.gif" border="0" align="middle" />).
I would do it like that :
- every X milliseconds, set a "Starting Point" var as the current (Touch.X, Touch.Y)
- X milliseconds, check the new (Touch.X1, Touch.Y1)
- do a check :
IF ((X1-X) > threshold, (X1-X) > 0, AND (Y1-Y) < variationOnAxis),
THEN Move 10 pixels to the right.
IF ((X1-X) < threshold, (X1-X) < 0, AND (Y1-Y) < variationOnAxis),
THEN Move 10 pixels to the left.
IF ((X1-X) < variationOnAxis, AND (Y1-Y) > threshold, (Y1-X) > 0),
THEN Move 10 pixels to the bottom.
IF ((X1-X) < variationOnAxis, AND (Y1-Y) < threshold, (Y1-X) < 0),
THEN Move 10 pixels to the top.
The idea is to 1) set a variation on the vertical or horizontal axis that allows the player to draw lines that aren't "exactly" vertical or horizontal, and 2) check if his finger has moved farther than a pre-determined threshold in one of those direction.