If you would like your custom visualization to be usable in other apps on your Splunk server, you must make the visualization global.
To enable the visualization to be global, in the $SPLUNK_HOME/etc/apps/ operational_intelligence/metadata directory create or update the local.meta file with:
[visualizations/tag_cloud] export = system
The export = system setting tells Splunk that the tag_cloud visualization should be made available to all other apps on the server.