code: public void vertex(double x, double y) {
          MODE = LINES;
          this.cornerX.add(x);
          this.cornerY.add(y);
          this.cornerZ.add(0d);
          setNumberOfCorner(this.cornerX.size());
          calcG();
      }
docs: Adds the corner of Polygon.
  
  @param x The x-coordinate of a new added corner.
  @param y The y-coordinate of a new added corner.