code: func (dc *Context) RotateAbout(angle, x, y float64) {
  	dc.Translate(x, y)
  	dc.Rotate(angle)
  	dc.Translate(-x, -y)
  }
docs: // RotateAbout updates the current matrix with a clockwise rotation.
  // Rotation occurs about the specified point. Angle is specified in radians.