So you've decided to create your own SSRS 2008 charting control. Good luck. The API is highly under-documented and there are very few examples out there. Now that I've got that disclaimer out of the way I'll proceed.
One of the decisions you'll have to make is how to structure your data for consumption by your control.
If your chart only needs a simple mapping scheme, where you consume data as it's returned by the dataset, then this tip won't matter much to you.
If you want to do something more complex then you'll be pleased to discover that any custom chart has similar column and row grouping capabilities that a standard tablix has.
When getting your control databinding working create a tablix for debugging. This tablix should have the same grouping setup, including expressions, which also consumes the same dataset. This way you can debug any issues with the data you're receiving and also experiment with new grouping setups and expressions without wondering if there's a problem with your code.
I'm not going to go into great depth about the databinding code itselfu at this stage. I plan to do that in a future post.