So the general state machine behind displaying those trends starts up in a "Get initial data from the server state", and while it's in that state it just displays the "Loading..." bit. Once it gets data back it renders the chart. If it's stuck at "Loading...", that means the server isn't returning any data to render.
As far as why there's no data to render, I'm not sure. We had a bug early on where it could take a long time for the first point for the data set with the longest interval we're storing to show up, and that led to being stuck at Loading..., but I'm pretty sure we squashed that before release, and besides that, we don't try and load that interval at page load time anyway.