function set( fmt ) {
	/* eslint-disable no-invalid-this */
	if ( !isNull( fmt ) && !isString( fmt ) ) {
		throw new TypeError( format( 'invalid assignment. `%s` must be a string or null. Value: `%s`.', 'xTickFormat', fmt ) );
	}
	if ( fmt !== this._xTickFormat ) {
		debug( 'Current value: %s.', this._xTickFormat );

		this._xTickFormat = fmt;
		debug( 'New value: %s.', this._xTickFormat );

		this.emit( 'change' );
	}
}
Sets the x-axis tick format.

@private
@param {(string|null)} fmt - axis tick format
@throws {TypeError} must be a string