tension (tension) {
        logger.warnOnce('dc.lineChart.tension has been deprecated since version 3.0 use dc.lineChart.curve instead');
        if (!arguments.length) {
            return this._tension;
        }
        this._tension = tension;
        return this;
    }
Gets or sets the tension to use for lines drawn, in the range 0 to 1.

Passed to the {@link https://github.com/d3/d3-shape/blob/master/README.md#curves d3 curve function}
if it provides a `.tension` function. Example:
{@link https://github.com/d3/d3-shape/blob/master/README.md#curveCardinal_tension curveCardinal.tension}.

This function exists for backward compatibility. Use {@link LineChart#curve}
which is generic and provides more options.
Value set through `.curve` takes precedence over `.interpolate` and `.tension`.
@deprecated since version 3.0 use {@link LineChart#curve} instead
@see {@link LineChart#curve}
@param  {Number} [tension=0]
@returns {Number|LineChart}