You know, if you're already on the command line, gnuplot and github already provide ~similar functionality. I can do the same in ~the same number of keystrokes if things are aliased properly. Also, keeping things local is nice.
I use gnuplot to work with large datasets I extract from telco carrier logs (for client usage analysis). It's one of the few tools you can throw a 100,000 point dataset at and it graphs it in seconds. Try that with most scripting language graphing libraries, and you'll be there for a few minutes.
The problem I have with gnuplot -- and maybe I'm the limitation, not the software -- is that I find it difficult to use, and the result of some input is inconsistent. Not to appear self-deprecating, but I really do think I might be the limitation here. Regardless of that, I often waste time trying to figure out why gnuplot is behaving as it is.
I have no idea if chartulo.us is going to choke on my datasets, but I hope I'll be able to use it. I really could use a tool that is a little easier to use than gnuplot.
Tableau does nearly instant plotting of millions of data points with a nice user interface; it may be worth checking out if your job depends on visualizing massive amounts of data.
Nothing fancy. Either a shared folder in my home-directory for users on the same machine, or email. Most of my work is separate individuals showing off their data rather than working together on one dataset. That could be a cool feature though--project groups or something.
