Can anyone recommend a library for chart generation (bar charts, pie charts etc.) which runs on both Java and .Net?
Have you looking into using JFreeChart. I have used it on a few Java projects and its very configurable. Its free but I think you can purchase the developers guide for $50. Its good for quick simple charts too. However performance for real-time data is not quite up to par (Check out the FAQ).
They also have a port to .NET however I have never used it.
Hope that helps.
ChartDirector is fantastic and supports more than just Java and .NET.
Dundas Charts was about the easiest thing ever to get up and producing amazing looking charts.
ChartFX (http://www.softwarefx.com) has been a leader in charting for years. I personally have used several different versions for over 8 years and it is rock solid.
I have re-evaluated charting options periodically, and ChartFX has won in my environment based almost purely on feature set. It is not free or cheap, but it is well worth the price they charge.
-Geoffrey
Here is a belated answer:
Use the Google Chart API. It will allow you to create charts in a programming language and platform agnostic way -- assuming your app will have an Internet connection at all times. Use it in combination with .Net and Java wrapper APIs that you can find here.
I wrote one: charts4j.