yAxisPadding (padding) {
        if (!arguments.length) {
            return this._yAxisPadding;
        }
        this._yAxisPadding = padding;
        return this;
    }
Set or get y axis padding for the elastic y axis. The padding will be added to the top and
bottom of the y axis if elasticY is turned on; otherwise it is ignored.

Padding can be an integer or percentage in string (e.g. '10%'). Padding can be applied to
number or date axes. When padding a date axis, an integer represents number of days being padded
and a percentage string will be treated the same as an integer.
@param {Number|String} [padding=0]
@returns {Number|CoordinateGridMixin}