Hi Laurence Fossati,
Let me start with that you need to upgrade with the latest internal build from your account (version 1213) because we fixed an issue with the selection of scatter series there.
Unfortunately we do not provide build-in functionality which maintains selection of legend items which are not auto generated. Attached you can find a project where such behavior is achieved but it is one-way. I mean that if you select series in the chart area, the legend item with the same index is highlighted, but selecting a legend item does not select the corresponding scatter series.
I hope that the sample application will help you in gaining your object.
the Telerik team
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