I created simple image based on OOUI icons and uploaded here https://commons.wikimedia.org/wiki/File:Graph-fallback.svg

See also https://design.wikimedia.org/style-guide/visual-style_icons.html

I changed design to be similar to visual style illustrations guide and also added 1,6 ratio version.

I like this new version a lot better than the first design, because it has a bit more details and looks less like an icon. I think it works well at some of the larger sizes that these graphs occupy on a page.
I'm not entirely sure about the danger triangle however.. We are strongly signalling 'error', while that isn't necessarily the case. The graph has not loaded, not yet loaded or is not expected to load.. that are three very different reasons to show this image, not all requiring an error indication. Maybe just not having the triangle would be better ?

Instead, maybe we should fatten up the lines a bit ? Turn it back a little bit more towards a stylistic design of the first image, making it more difficult to confuse it with an actual graph, but still with more details than the first idea ?
Googling placeholder graph/chart brings up some inspiration perhaps... https://www.google.com/search?q=placeholder+graph&tbm=isch

Thank you for doing this @Pietrasagh !

Thank you for comment. I read in guidelines they like to have more "curvy" shapes so I changed it a bit and added bars. I think it look more "cartoony" and would be hard to confuse with real graph. I also left previous version with triangle. Maybe it could be use in case of chart error.
I think that my illustration skills are now somewhere on spectrum between toddler and Picasso ;-)
https://commons.wikimedia.org/wiki/File:Graph-bar-line-fallback-illustration.svg

Love this version ! Exactly what I had in mind.

Added via I3465888bb5a3475e82b7a0899bba151c4b029f23

https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Graph/+/909758/