Seems like a user's selected theme is partially applied to graph API requests (checked also in current r13671).

If the authenticated Observium user has a dark theme selected, the legend text on API graphs changes to white but the background is left as the default white, so you get invisible legend text.

I could see the argument for (not) applying the authenticated user's selected theme to API requested graphs (perhaps it could be an argument, or even passing custom colours as arguments), but half-applying it leaving elements invisible seems likely to be unwanted behaviour.