drawBall(x, y, elementName) {
    let r = this.opts.bondLength / 4.5;

    if (x - r < this.minX) {
      this.minX = x - r;
    }

    if (x + r > this.maxX) {
      this.maxX = x + r;
    }

    if (y - r < this.minY) {
      this.minY = y - r;
    }

    if (y + r > this.maxY) {
      this.maxY = y + r;
    }

    let ball = document.createElementNS('http://www.w3.org/2000/svg', 'circle');
    ball.setAttributeNS(null, 'cx', x);
    ball.setAttributeNS(null, 'cy', y);
    ball.setAttributeNS(null, 'r', r);
    ball.setAttributeNS(null, 'fill', this.themeManager.getColor(elementName));
    this.vertices.push(ball);
  }
Draw an svg ellipse as a ball.

@param {Number} x The x position of the text.
@param {Number} y The y position of the text.
@param {String} elementName The name of the element (single-letter).