[ixpmanager] grapher display bug and fix

[Athanasios Douitsis]

Hello,

At GR-IX, we've noticed that under some conditions, the member
statistics page will display duplicate diagrams (ie, the same client
graph twice). Upon a trifle of investigating, we've noticed this is a
very simple bug and, even better, even simpler to fix. Here is the pull
request:

https://github.com/inex/IXP-Manager/pull/480/commits/a6609437b2b213aea0bf222b7975eda265d38ddc

Could someone pray take a quick look at it? We can certainly apply our
own private patches, but we vastly prefer this is fixed upstream.

Kind regards,
-- 
Athanasios Douitsis
National Technical University of Athens NOC
e: aduitsis at noc.ntua.gr | t: +302107722409