function set( fmt ) {
	/* eslint-disable no-invalid-this */
	var err = isValid( fmt );
	if ( err ) {
		throw err;
	}
	debug( 'Current value: %s.', this._tickFormat );

	this._tickFormat = fmt;
	debug( 'New Value: %s.', this._tickFormat );

	this.emit( CHANGE_EVENT );
}
Sets the axis tick format.

@private
@param {(null|string|Function)} fmt - tick format
@throws {TypeError} must be either null, a string, or a function