In order to measure the oriented-angle from OA vector towards OB vector, click on icon.
Click on A, O then B.
If the unity angle of the figure is degree, this measure will lie in the range -180 (excluded) to 180.
If the unity angle of the figure is radian, this measure will lie in the range - π (excluded) to π.
The three points clicked must be named.
If they aren't they will be named by the tool.
MathGraph32 will ask you a name for this measure.