MediaWiki 1.30.0 (d0c8f49)
PHP 7.2.1-1+ubuntu17.10.1+deb.sury.org+1 (fpm-fcgi)
composer require mediawiki/graph-viz '*' creates extensions/GraphViz folder yet doesn't enable the GraphViz extension for MediaWiki. It doesn't appear in Special:Version and the graphs remain source code.
wfLoadExtension ('GraphViz'); helps but this is not the way Composer is supposed to work.
Also, if I try to alter GraphViz setting after enabling the extension (e.g. $wgGraphVizSettings->execPath = '/usr/bin/';), PHP issues a warning:
Creating default object from empty value in (path to MediaWiki)/LocalSettings.php on line (line number).
Obviously, the $wgGraphVizSettings object is not instantiated at this point; I presume I should do it in some hook, but it is not documented at the extension's page at mediawiki.org.