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

	this._scale = fcn;
	debug( 'New Value: %s.', this._scale );

	this.emit( CHANGE_EVENT );
}
Sets the scale function.

@private
@param {Function} fcn - scale
@throws {TypeError} must be a function