defined (defined) {
        if (!arguments.length) {
            return this._defined;
        }
        this._defined = defined;
        return this;
    }
Gets or sets a function that will determine discontinuities in the line which should be
skipped: the path will be broken into separate subpaths if some points are undefined.
This function is passed to
{@link https://github.com/d3/d3-shape/blob/master/README.md#line_defined line.defined}

Note: crossfilter will sometimes coerce nulls to 0, so you may need to carefully write
custom reduce functions to get this to work, depending on your data. See
{@link https://github.com/dc-js/dc.js/issues/615#issuecomment-49089248 this GitHub comment}
for more details and an example.
@see {@link https://github.com/d3/d3-shape/blob/master/README.md#line_defined line.defined}
@param  {Function} [defined]
@returns {Function|LineChart}