WDQS is fine now, so probably not that. Also I see the queries work fine on the demo page. In any case, static images are supposed to be cached, so they should be showing up even when WDQS is down.
On https://www.mediawiki.org/wiki/Extension:Graph/Demo/Sparql , "Largest Cities" image only loads for me after bypassing the browser cache.
Now does that make this task solved or not? :P