good evening ... it's late here and i hope i don't give you a bad recommendation ;) ... but i faced that problem once and looked for something that does the job you want ... there are some libraries out there ... google for them ... i remember plotkit for instance:
http://www.liquidx.net/plotkit/
or simple google 'charts with javascript' for example
http://www.developertutorials.com/scripts/7/134/
a lot of them seems to do very well ... but i think you should choose the one you like ... the time i needed to use that, i only needed bargraphs ... that is relativly easy to do by yourself ... so i wrote my own barchart-script ... it did only simple vertical or horizontal barcharts ... and was not able to do some things that you need for a real cool lib ... like simple (generic) creation, simple passing of data ... ideally in native js-array/object notation ... all together: you may find a lot of scripts out there that will help you ... and some produce real slick output :) ... if you prefer to 'selfcode', it would be possible too ...
but remember ... javascript doesn't have the ability to produce bitmaps ... all 'javascript-graphics' are limited to the creation of dom-elements (divs, lines, spans together with styles etc.) ... that gives you the advantage of creating interactive charts (attach event-handlers to a bar - a div with width and height) ... but no bitmap ... thats not possible ... i think ;)