How do you rotate a figure 90 degrees counterclockwise about the origin?

Given a set of points, (x1, y1), (x2, y2), etc.

Take the absolute value of each point's x and y values, and replace those.

Take the inverse point of each point, e.x. (x1, y1) -> (y1, x1)

Apply the signs that correspond to the quadrant counterclockwise of the quadrant the point was in.

e.x. (3, 5) is in the First Quadrant. The Second Quadrant is counterclockwise of the First, so we will have the x-value of the point negative: (-3, 5).

Do that for all points.