Too many data sources on one graph causes HTTP 414 and empty graphs

[EvanKrall]

I find it useful, occasionally, to generate firefly graphs with many lines on them - say, one per server.

Since every single data source on a graph is stuffed into the same URL, this creates really really long URIs, and when the Firefly UI goes to fetch these,

Failed to load resource: the server responded with a status of 414 (Request-URI Too Large)

#55 would help me avoid putting too many data sources in one graph. (but wouldn't entirely fix this, since occasionally I want to use a large number of sources that don't have a simple wildcard match.)