I think, the best is jQuery Flot charts, he has a good documentation (could be better) and even an amazing tutorial site (that I think is better to understand and use. the Flot chart)
Also, he has over 1.2K questions on SO. And plugins like the one for Zoom/Navigation and tons of others plugins for it
Highcharts is a great option too, with an excellent documentation and demo pages.
I don't like the Sparklines that much, but is a good and "easy to use" plugin.
And you can always take a look at the jQuery site page of plugins for graphics
Where you can always take a look in the latest list with votes and stuff.
