Can't seem to understand diagram cells

[BenRacicot]

I'm simply trying to get diagram cells to work. I've read all the documentation but none of the examples output a diagram. Editor, preview and side-by-side still show the text and no chart. Perhaps it's the built-in theme "Dark" I'm using, otherwise there must be something obvious I'm missing.

[ylian]

The problem is that there are a few subtypes in the diagram cell. Note in the toolbar at the top, you have selected Flowchart. The diagram you have is a sequence diagram.