function get() {
	/* eslint-disable no-invalid-this */
	if ( isNull( this._xTickFormat ) ) {
		return this._xTickFormat;
	}
	if ( this._xScale === 'time' ) {
		return timeFormat( this._xTickFormat );
	}
	return format( this._xTickFormat );
}
Returns the x-axis tick format.

@private
@returns {(Function|null)} format function or null