http://www.highcharts.com is amazing and used across a lot of the US's biggest investment banks. Creative commons version available or per user/organizational licenses.
http://d3js.org though I have a particular soft spot for.
http://d3js.org though I have a particular soft spot for.
Their API reference is pretty good as well: http://api.highcharts.com/highcharts
For our dashboards and most reports we now use a combination of custom Highcharts charts and standard widgets available at Geckoboard.