@Niko_Izsak
I ran in the same issue with my RPi 3. After debugging it a bit it seems you need PhantomJS installed on the RPi 3 to get rendered images. A solution is provided by fg2it in the InfluxDB+Grafana topic. I haven’t tried the solution myself yet.
The GitHub page for the PhantomJS packages is: GitHub - fg2it/phantomjs-on-raspberry: Phantomjs for raspberry pi (armv6/armv7) and aarch64/arm64
In my case sudo cat /var/log/grafana/grafana.log showed following:
t=2016-12-17T18:34:02+0100 lvl=eror msg="Failed to render to png" logger=context userId=0 orgId=1 uname= error="PhantomRenderer::renderToPng timeout (>30s)"
After searching on the error message the following Grafana issue tells the root cause may be the missing PhantomJS:
https://github.com/grafana/grafana/issues/2662