_debug(ctx, color) {
    if (this.barnesHutTree !== undefined) {
      ctx.lineWidth = 1;

      this._drawBranch(this.barnesHutTree.root, ctx, color);
    }
  }
This function is for debugging purposed, it draws the tree.
@param {CanvasRenderingContext2D} ctx
@param {string} color
@private