'Adds a skew in place.
*xShear* and *yShear* are the shear angles along the *x*- and
*y*-axes, respectively, in radians.
Returns *self*, so this method can easily be chained with more
calls to :meth:`rotate`, :meth:`rotate_deg`, :meth:`translate`
and :meth:`scale`.'

def skew(self, xShear, yShear):

    rotX = np.tan(xShear)
    rotY = np.tan(yShear)
    skew_mtx = np.array([[1.0, rotX, 0.0], [rotY, 1.0, 0.0], [0.0, 0.0, 1.0]], float)
    self._mtx = np.dot(skew_mtx, self._mtx)
    self.invalidate()
    return self
