The difference between using the two approaches is that SeriesMapping allows for configuring a data object, which is mapped to the Chart and in this scenario RadChart itself creates the DataPoints, whereas using DataSeries requires setting the DataPoints manually.
In general, we would strongly recommend using SeriesMapping, unless you need a specific customization that requires setting DataPoints manually. One such scenario could be if you're creating a threshold line - you may configure a Line series using DataSeries, then setting two or more DataPoints with the same values and thus simulating a threshold line.
Hope this helps.
the Telerik team
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