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Polyfit, polyval and plot

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Jovos
Jovos el 16 de Feb. de 2016
Comentada: Jovos el 18 de Feb. de 2016
I am trying to fit the polynomials to the data with 0,1,2 degrees respectively and plot them on the same graph. I wonder why there is only one line shown on the plot which is the 0 degree polynomial.
y = flip(AdjClose).'
x = 1:503;
polyfit(y,x,0);
coef1 = polyfit(y,x,0);
y1 = polyval(coef1,x);
hold on
plot(x,y);
plot(x,y1);
polyfit(y,x,1);
coef2 = polyfit(y,x,1);
y2 = polyval(coef2,x);
plot(x,y2);
polyfit(y,x,2);
coef3 = polyfit(y,x,2);
y3 = polyval(coef3,x);
plot(x,y3);

Respuesta aceptada

the cyclist
the cyclist el 16 de Feb. de 2016

Because the syntax should be

polyfit(x,y,N)

and not

polyfit(y,x,N)

Here is some code that illustrates the fix:

x = 1:503;
y = 250 + 0.02*x + 0.005*x.^2 + 0.2*rand(1,503).*x;
figure
polyfit(y,x,0);
coef1 = polyfit(x,y,0);
y1 = polyval(coef1,x);
hold on
plot(x,y,'.');
plot(x,y1);  
polyfit(x,y,1);
coef2 = polyfit(x,y,1);
y2 = polyval(coef2,x);
plot(x,y2);
polyfit(y,x,2);
coef3 = polyfit(x,y,2);
y3 = polyval(coef3,x);
plot(x,y3);
  5 comentarios
the cyclist
the cyclist el 17 de Feb. de 2016
It's because your last y value is NaN, which is causing the fitted coefficients to all be NaN, and therefore unplottable.
Put the lines
x(end) = [];
y(end) = [];
in your code before the first fit, and you will see results.
Jovos
Jovos el 18 de Feb. de 2016
Very nice!

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Más respuestas (1)

Walter Roberson
Walter Roberson el 17 de Feb. de 2016
When you use "hold on" before you have done any plotting, your xlim and ylim are frozen at their default values, which are [0 1] and [0 1]. If your plotting goes outside that range then nothing will show up. You should delay the "hold on" until after the first plot is done.
  1 comentario
Jovos
Jovos el 17 de Feb. de 2016
I don't think that makes a change.

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