Hi Thomas,
Thanks for your interest in the Syncfusion Flutter chart widget. Your requirement can be achieved using the SplineArea series. As of now it is not available, but we already have this in our roadmap, and it has been implemented. This is in the testing phase and it will be available in volume 4 main release which is expected to be out in the middle of December 2019. Here we are attaching the expected output of the spline area series.
Thanks,
Dharani.