code: function (x, y) {
              var a = this.val,
                 _x = x,
                 _y = typeof(y) === "undefined" ? _x : y;
  
              a[0] *= _x;
              a[1] *= _x;
              a[3] *= _y;
              a[4] *= _y;
  
              return this;
          }
docs: scale the matrix
  @name scale
  @memberOf me.Matrix2d
  @function
  @param {Number} x a number representing the abscissa of the scaling vector.
  @param {Number} [y=x] a number representing the ordinate of the scaling vector.
  @return {me.Matrix2d} Reference to this object for method chaining