Spline fitting involves using a continuous curve to approximate or compare the functional relationship between the coordinates represented by discrete points on a plane.

1) Select the "Spline Fit" tool  or "Curve" in the "Graph Center" in the sidebar , and click on the canvas to place the first point.

2) Move the mouse and click to place the second point, creating a line between the points.

3) Continue moving the mouse and click to place the third point, forming a smooth cruve segment that bends according to the point's positions.

4) Click the "Main Selection" tool to complete the curve drawing, or continue adding points to create a more complex curve.

Note: Points must be added in sequence, with the second point after the first, the third point must be in front of the second point, and no three points on the same vertical line.

Modify the curve, use the "Selection" tool to click and move points on the line.

For exact curve fitting, use Scatter Plot.