MATLAB: Linear fit of a semilog graph

Question

Hello,

I have my data as follows with F1, F2, F3, N1, N2 and N3. I want to do a linear fit of my data and plot that. I tried polyfit as seen in my code. But it just plots a horizontal line…? I tried adding my vectors to a Ntot and Ftot as seen.

What should I do? Thank you.

F1=[200 200 200];

N1=[10000 15000 20000];

F2=[300 300 300];

N2=[8000 7000 7500];

F3=[100 100 100];

N3=[120000 140000 80000];

Ftot=[F3 F2 F1];

Ntot=[N3 N2 N1];

figure

semilogx(N1,F1,'o')

hold on

semilogx(N2,F2,'o')

semilogx(N3,F3,'o')

xlabel('N');

ylabel('F');

grid on

P = polyfit(Ntot,Ftot,1);

    yfit = P(1)*Ftot+P(2);
    plot(Ntot,yfit,'r-.');

xlim([1000 500000])

ylim([0 max(Ftot)+50])

Answer 1

You should sort the values in

Ntot

using

[Ntot,i] = sort(Ntot);

and sort

Ftot

accordingly using

Ftot = Ftot(i);

Then you can do the

polyfit(...)

and as Torsten mentions, you should use

yfit = P(1)*Ntot+P(2);

Then plot it using

semilogx(Ntot,yfit,'r-')

although

plot(...)

also works. If you want the semilogarithmic plot to have a straight line, then use

log10(Ntot)

instead of

Ntot

in the line containing the

polyfit(...)

and the line containing

yfit = ...