We have reviewed your source code (at least partially) and do not see how it can violate our EULA. If I understand correctly, you just write descendants for our controls and do not use our own code. Thus, you are free to sell these products to our customers or give them charts for free. Moreover, I think that you are doing a great job creating such custom extensions to our products and both you and DevExpress will benefit from xdxCharts.
The only problem I see is the fact that we cannot tell you whether a certain customer is our subscriber or not.
So, it may be necessary to change your business model in the future.