code: public Matrix4d rotationTowardsXY(double dirX, double dirY) {
          if ((properties & PROPERTY_IDENTITY) == 0)
              this._identity();
          this.m00 = dirY;
          this.m01 = dirX;
          this.m10 = -dirX;
          this.m11 = dirY;
          properties = PROPERTY_AFFINE | PROPERTY_ORTHONORMAL;
          return this;
      }
docs: Set this matrix to a rotation transformation about the Z axis to align the local <code>+X</code> towards <code>(dirX, dirY)</code>.
  <p>
  The vector <code>(dirX, dirY)</code> must be a unit vector.
  
  @param dirX
  the x component of the normalized direction
  @param dirY
  the y component of the normalized direction
  @return this