The angle of this offset as radians clockwise from the positive x-axis, in
the range -pi to pi, assuming positive values of the x-axis go to the
left and positive values of the y-axis go down.
Zero means that dy is zero and dx is zero or positive.
Values from zero to pi/2 indicate positive values of dx and dy, the
Values from pi/2 to pi indicate negative values of dx and positive
values of dy, the bottom-left quadrant.
Values from zero to -pi/2 indicate positive values of dx and negative
values of dy, the top-right quadrant.
Values from -pi/2 to -pi indicate negative values of dx and dy,
the top-left quadrant.
When dy is zero and dx is negative, the direction is pi.
When dx is zero, direction is pi/2 if dy is positive and -pi/2
if dy is negative.
- distance, to compute the magnitude of the vector.
- Canvas.rotate, which uses the same convention for its angle.