cairo:rotate (cr angle)

cr -- a cairo:context-t instance
angle -- a number coerced to a double float for an angle in radians by which the user-space axes will be rotated

Modifies the current transformation matrix (CTM) by rotating the user-space axes by angle radians. The rotation of the axes takes places after any existing transformation of user space. The rotation direction for positive angles is from the positive x axis toward the positive y axis.