I believe that this is expected result for such data. The spline will make a smooth transition and the actual point must be included in the line, this is why it goes below zero. You can adjust this with the SplineTension property but still, the line will go below zero. I have tested this in excel and I am getting the same results (see attached).
Let me know if you have additional questions.
Regards,
Dimitar
Progress Telerik
Get quickly onboarded and successful with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers.Learn More.