Hi Martin,
Greetings from Syncfusion.
We have validated your query and you can achieve this using the StepLineSeries.onCreateRenderer callback. You will get this callback when creating the state for the StepLineSeries. Override the getStrokePaint() method in the custom StepLineSegment class and set the strokeCap as StrokeCap.round. This makes the line edges rounded.
We have attached the sample for your reference. Please check it and let us know if you need any further assistance with this.
Regards,
Praveen G.
Attachment:
flutter_charts_b0be6a25.zip