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

	this._label = label;
	debug( 'New value: %s.', this._label );

	this.emit( CHANGE_EVENT );
}
Sets the axis label.

@private
@param {string} label - axis label
@throws {TypeError} must be a string