How to create a best fit line here?

I was trying to make a best-fit line with the data that I have, but for some reason the polyfit command doesn't seem to give me the right answer for it. Does anyone know how to fix it? Thank you.

thickness = 0.8; % the units are in mm
width = 12.7;
area = thickness*width;

load = xlsread('homework1.xlsx','A9:A42');
strain = xlsread('homework1.xlsx','B9:B42');

trueStress1 = (load.*(1+strain))./area; % true stress calculation 
trueStrain1 = log(strain + 1); % true strain calculation 
x = log(trueStrain1);
y = log(trueStress1);
p = polyfit(x,y,1); % bestfit the line between points 
f = polyval(p,x); 

figure;
plot(x,y,'o');
hold on
plot(x,f,'-');
title('True Stress vs. True Strain');
xlabel('Log \epsilon');
ylabel('Log \sigma');
legend('data','linear fit','location','northwest')

Answer 1

[Chad Greene]

It looks like the first x value is -Inf. Try eliminating that point from the polynomial fit, like this:

p = polyfit(x(2:end),y(2:end),1); % bestfit the line between points 

[Zifeng Qiu]

Thanks, it worked