linear fit of a semilog graph

Daniel (view profile)

on 19 Nov 2014
Latest activity Commented on by Daniel

Daniel (view profile)

on 19 Nov 2014
Accepted Answer by Erik

Erik (view profile)

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])

Torsten

Torsten (view profile)

on 19 Nov 2014
According to your call to polyfit, the next line must read
yfit = P(1)*Ntot+P(2);
yfit = P(1)*Ftot+P(2);
Best wishes
Torsten.
Star Strider

Star Strider (view profile)

on 19 Nov 2014
The other option, of course, is to use the polyval function.

on 19 Nov 2014
Edited by Erik

Erik (view profile)

on 19 Nov 2014

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)
Ntot
in the line containing the
polyfit(...)
and the line containing
yfit = ...

Daniel

Daniel (view profile)

on 19 Nov 2014
Thank you. Okey, but now I have a plot like the attached picture.
Can I do so a straight line goes from the data at 300 at the y-axis to the data at 100 on the y-axis?
Is it some way to do an approximation for that? 