Polar coordinate plots are by default oriented such that theta = 0 is on the
right side of the plot, with theta increasing as you proceed counterclockwise
so that theta = 90 degrees is at the top. set theta allows you to change
the origin and direction of the polar angular coordinate theta.
set theta {right|top|left|bottom}
set theta {clockwise|cw|counterclockwise|ccw}
unset theta restores the default state "set theta right ccw".
