I wish to subscribe to Joplin Cloud but don't know whether the cloud version supports plugins. Possible to install plugins to Joplin Cloud? for example Graph plugin.
You install plugins on your Joplin client; Joplin cloud syncs your notes between devices, so any notes that utilize Graph synchronize between devices. Note that plugins are only available on the desktop clients.
i thought there was a webapp version running in browser
To my knowledge, there are no plugins that run directly on the server side. So your web app probably runs from the desktop as well. You should have no issues.
yah it's webview, but a webapp version may use something similar such as iframe