Unless this has changed, you need the $9,950/year plan to serve it "behind your own firewall". I.e. otherwise plots on done on their systems by uploading data.
That used to be the case until the end of last year, but it has been open-sourced and you can render a plot entirely on the client side, without a roundtrip to Plot.ly's servers (no rate limits anymore, custom integrations to your existing stuff, etc.), for free.
We looked into it after they open sourced it; AFAIK, it is still designed to connect to their streaming servers with an API key (or run locally in an ipython notebook or similar "offline"). If you want to make your own private dashboards for your own system you still need https://plot.ly/product/enterprise/#plotly-on-premise
