Polyfit, polyval and plot

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);