In a chart that displays lines (Line type or XY type), you can choose to show curves instead of straight lines. Some options control the properties of those curves.
Select Cubic Spline or B-Spline.
These are mathematical models that influence the display of the curves. The curves are created by joining together segments of polynomials.
Optionally set the resolution. A higher value leads to a smoother line.
For B-spline lines optionally set the degree of the polynomials.