The date axis can be configured to behave in the manner you describe using the maxDateGroups option:
It will aggregate the data in such time intervals that the total number of points remains below [maxDateGroups]. The intervals it chooses from are defined in autoBaseUnitSteps.
If you're dealing with a lot of data points (say 10 000) you should consider doing aggregates on the server. We do this in the stock chart virtualization demo.
The server-side grouping code is located in the ASP.NET MVC package:
In fact, the stock chart is an extension built on top of the regular chart widget. It does not limit you to financial series and will happily plot line charts as well. You can use it directly if you need interactive selection.
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