MQTT data not showing up in datasource

I’m using Grafana v2.9.3 with the MQTT datasource plugin. The broker is hosted on the same server and is accessible on standard port 1883 without credentials (allow anonymous login = True).
The MQTT datasource confirms a succesful connection and when opening the Explore page it does appear to try and stream data (given the blue scrolling line above the table).

However: No data will ever show up, no matter what I post in the selected topic. Does the data require a specific format? If so what would that be?

My mosquitto broker is not the problem: I can succesfully stream and plot data from my broker with a instance.

Somehow my own grafana server can connect to the mqtt server, but not receive data. What is it that could block the data from arriving?