Data Join function currently support link your data and layer, but it don't support calculate the data like total sales after link. Our developer think it's a good idea and we will implement that in future.
But class break style cannot shows correct because after data join, the data should be converted to string, so you get the warning to ask for only numeric data. Why we did that because if our layer link to different data source, our main purpose is view same column data from different source.
We did some discuss about your requirement and think it won't works by UI in current version, but if in code we can do more thing.
Our developer want to know more about your progress, could you have more detail description about that? We want to make sure you have what data source, how to deal with them and show what as the result. So we can provider better suggestion for that.