Ask HN: Good javascript graphing libraries

Quick query from a long time lurker... I'm working on an app that helps people make informed financial decisions. As part of this I need to draw graphs to help users visualise the data. I'm currently using the google charts javascript library. However I'm slightly troubled by relying on a company's ongoing support and traffic issues if my site ever takes off (http://code.google.com/apis/visualization/terms.html). What other libraries have readers had success with? I'm not looking to do anything too advanced, think along the lines of bar and line charts with annotations but I want it to work with older browsers (ie6). Thanks.