function Exports(ctx) {
      _classCallCheck(this, Exports);
      this.ctx = ctx;
      this.w = ctx.w;
    }
User possibly set string categories in xaxis.categories or labels prop
Or didn't set xaxis labels at all - in which case we manually do it.
If user passed series data as [[3, 2], [4, 5]] or [{ x: 3, y: 55 }],
this shouldn't be called
@param {array} ser - the series which user passed to the config