code: function skew(ax, ay) {
      return {
          a: 1, c: tan(ax), e: 0,
          b: tan(ay), d: 1, f: 0
      };
  }
docs: Calculate a skew matrix
  @param ax Skew on axis x
  @param ay Skew on axis y
  @returns {{a: number, b: number, c: number, e: number, d: number, f: number}} Affine matrix