[ixpmanager] Grapher with MRTG shows broken graphs

[Tom Duff]

Hi all,

I'm setting up IXP Manager V4.7.3 on Ubuntu 16.04 (I used the neat
bootstrap script, thinks for that!), and I've got my switch added.
I've followed the instructions to configure MRTG as the grapher
backend here:

       http://docs.ixpmanager.org/features/grapher/

I have MTRG running as a daemon (by the way, the script on that page
that performs the syntax check on the mrtg.cfg file has a small typo
in it, but that's been reported in bug 396 on github) - I'm seeing
mrtg running, and the rrd files are being generated in /srv/mrtg.
However, IXP Manager isn't loading graphs when you load the pages, all
I can see is broken images, which look like this in the source code:

[img width="100%" border="0" src="data:image/png;base64," /]

Every graph is broken. There are no errors in MRTG's log file,
everything on the backend seems to be working just fine. The web
interface just doesnt seem to be rendering the graphs.

If I switch my GRAPHER_BACKENDS from "mrtg" back to "dummy", the dummy
graphs load fine. I'm pretty sure I haven't missed a step, is there
some reason IXPManager wouldn't be rendering the graphs?  Any help
would be greatly appreciated.

--Tom