# curve fitting: how to get the fitting curve?

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Yuji Zhang on 6 Dec 2013
Commented: Yuji Zhang on 9 Dec 2013
Hi everyone~
I did some curve fitting using curve fitting toolbox. But I don't know how to get the fitting curve. Can somebody help?
Also, in the future I want to do this using code. I'm reading the manual for spline fitting. http://www.mathworks.com/help/curvefit/construction.html
There seems to be lots of kinds of it. I want it shape-conserve, and must pass all the points. Which one should I use?
Any help is appreciated. Thank you!

John D'Errico on 6 Dec 2013
Edited: John D'Errico on 6 Dec 2013
Use pchip to create a shape preserving spline. ppval can evaluate it.
Yuji Zhang on 9 Dec 2013
Hi Thanks John~
I did testing and found "spline" is more shape-preserving. See the example.
Do you know how I can control the curve to go up or down at the side where there's no data? Any discussion's appreciated. Thank u~
Example: circles: data (4 points)
blue curve: pchip
red curve: spline
code:
x0 = [-10 -5 0 5 ];
y0 = [-50 -10 0 -10];
x = -50:0.1:50;
y1 = pchip (x0, y0, x);
y2 = spline (x0, y0, x);
plot(x0, y0, 'o');
hold on;
plot(x, y1,'b');
plot(x, y2,'r');
Yuji Zhang on 9 Dec 2013
Hi John~
Actually too bad. For my case, "pchip" doesn't work for the left end, and "spline" doesn't work for the right end. I just want a bell shape facing down.
See the plot:
Code:
x0 = [-10 -5 0 5 100];
y0 = [-50 -10 0 -10 -200];
x = -300:0.1:300;
y1 = pchip (x0, y0, x);
y2 = spline (x0, y0, x);
plot(x0, y0, 'o');
hold on;
plot(x, y1,'b');
plot(x, y2,'r');
ylim([-700 200]);
Let me know is there any way to fix it. Thanks a lot~