function getXAxisFormat() {
            if (xAxisFormatType === 'number') {
                return d3Format.format(xAxisFormat);
            } else {
                return d3TimeFormat.timeFormat(xAxisFormat);
            }
        }
Gets the formatter function for x-Axis based on `xAxisFormatType` setter
Applies `timeFormat` formatter function if custom `xAxisFormatType`
that is not equal to 'number' is provided
@return {function(String): String}
@private