I was looking at alternatives to Google Chart Tools and came across Highcharts JS, which is my new favourite charting API. It’s extremely customizable, fast, and free for personal, academic, and non-profit use(*). You can also include buttons to print or save each chart as an image. The charts are also optionably zoomable through click and drag. Finally, Highcharts has been exhaustively documented, complete with JSFiddles for almost every feature. Check out my example Highcharts here.
* Note: I’m obligated to point out that Highcharts is not available for free for commercial use. See here for pricing.