xAxisLabel (labelText, padding) {
        if (!arguments.length) {
            return this._xAxisLabel;
        }
        this._xAxisLabel = labelText;
        this.margins().bottom -= this._xAxisLabelPadding;
        this._xAxisLabelPadding = (padding === undefined) ? DEFAULT_AXIS_LABEL_PADDING : padding;
        this.margins().bottom += this._xAxisLabelPadding;
        return this;
    }
Set or get the x axis label. If setting the label, you may optionally include additional padding to
the margin to make room for the label. By default the padded is set to 12 to accomodate the text height.
@param {String} [labelText]
@param {Number} [padding=12]
@returns {String}