curve (curve) {
        if (!arguments.length) {
            return this._curve;
        }
        this._curve = curve;
        return this;
    }
Gets or sets the curve factory to use for lines and areas drawn, allowing e.g. step
functions, splines, and cubic interpolation. Typically you would use one of the interpolator functions
provided by {@link https://github.com/d3/d3-shape/blob/master/README.md#curves d3 curves}.

Replaces the use of {@link LineChart#interpolate} and {@link LineChart#tension}
in dc.js < 3.0

This is passed to
{@link https://github.com/d3/d3-shape/blob/master/README.md#line_curve line.curve} and
{@link https://github.com/d3/d3-shape/blob/master/README.md#area_curve area.curve}.
@example
// default
chart
    .curve(d3.curveLinear);
// Add tension to curves that support it
chart
    .curve(d3.curveCardinal.tension(0.5));
// You can use some specialized variation like
// https://en.wikipedia.org/wiki/Centripetal_Catmull%E2%80%93Rom_spline
chart
    .curve(d3.curveCatmullRom.alpha(0.5));
@see {@link https://github.com/d3/d3-shape/blob/master/README.md#line_curve line.curve}
@see {@link https://github.com/d3/d3-shape/blob/master/README.md#area_curve area.curve}
@param  {d3.curve} [curve=d3.curveLinear]
@returns {d3.curve|LineChart}