Polyfit, polyval and plot

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);

Answer 1

[the cyclist]

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);

[Jovos]

After I changed the code to the following, there still is not graph showing. Can you tell me where is wrong？

y = flip(AdjClose).'
x = 1:503;

polyfit(x,y,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(x,y,2);
coef3 = polyfit(x,y,2);
y3 = polyval(coef3,x);
plot(x,y3);

[the cyclist]

It's difficult to know the problem without being able to run your code. Can you post a *.mat file with the value of AdjClose?

[Jovos]

Here it is.

[the cyclist]

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]

Very nice!

Answer 2

[Walter Roberson]

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.

[Jovos]

I don't think that makes a change.