Polyfit with plot, semilogx, semilogy, and loglog scales.

So I have vector x and vector y, and I need to determine which scale fits the data.

x = 2004:2015;
y = [9214, 9271, 9320, 10413, 10930, 11242, 10873, 11801, 13245, 14728, 15463, 16297];

So I decided to use polyfit and use all the plots and let MATLAB show me which one best fits.

%Output:
subplot(1,3,1)
p1 = polyfit(x_new,y,1);
m1 = p1(1);
b1 = p1(2);
plot(x, y, 'o', x, m1.*x+b1), xlabel('years'), ylabel('sales'), title('years vs sales')

subplot(1,3,2)
p2 = polyfit(x_new,log(y),1);
m2 = p2(1);
b2 = 10^(p2(2));
semilogy(x, y, 'o', x, b2*10.^(m2.*x)), xlabel('years'), ylabel('sales'), title('years vs sales')
 
subplot(1,3,3)
p3 = polyfit(log10(x),log10(y),1);
m3 = p3(1);
b3 = 10^(p3(2));
loglog(x, y, 'o', x, b3.*x.^m3), xlabel('years'), ylabel('sales'), title('years vs sales') 

And this is the graph I am getting:

I think the first one is right, and I believe that the line shouldn't look like that in the second one, and the third is just not working for some reason!!!

Answer 1

[Prabhan Purwar]

Hi,

Try fitting a higher degree of a polynomial using polyfit function while fitting a curve to logarithmic data as it tends to be a curve rather than a straight line.

For more details refer to the following link

• https://in.mathworks.com/help/matlab/ref/polyfit.html (polyfit)

Hope it helps!!