There is a huge plethora of js charting libraries out there. I had used flot.js for Engadget's price graph, but it took some hackery to get it to work the way we wanted it to. This is a fairly newer one, that focuses on 6 of the most common charts and simplicity of use. There isn't currently any tooltips, but there is a pull request which does have them (and other features). There hadn't been much development on the repo for a while but the creator tweeted a WIP of responsive graphs with tooltips.