The Legend needs to be in the visual tree of the SciChartSurface in order for SciChartSurface.ExportToBitmapSource() to export it. There are several ways to achieve this:
I would personally recommend (1) as this is our supported API for printing a legend, and the easiest to achieve, but any of them will work!
For more info on the LegendModifier and SciChartLegend API see here.
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