How can I extend polyfit line on loglog scale ?

I wrote this code, but I need to extend this line from y=10 to y=100. I tried but I couldn't do it. Can anybody help ?

x= [0.310 0.220 0.125 0.065]
y= [95.4 63.3 36.8 22.4] 

X=log10(x); Y=log10(y);  
P=polyfit(X,Y,1);        
yhat=10.^[polyval(P,[X(1) X(end)])];  
loglog(x,y,'s')
hold on
loglog([x(1) x(end)],yhat) 
xlabel('(mm)')
ylabel('(%)')
grid on
slope = P(1)
txt = ['m=' num2str(P(1))]
text(0.4,40,txt)

Answer 1

[Bjorn Gustavsson]

When you calculate yhat you only do that for the first and last element of your x-values that you use for the fit. You should try to extend the range of x-values and calculate yhat-values for a larger range. Something like this might help you on your way:

x= [0.310 0.220 0.125 0.065]
y= [95.4 63.3 36.8 22.4] 

X=log10(x); Y=log10(y);
P=polyfit(X,Y,1);
Xi = log10(linspace(min(x)/10,max(x)*10),101); % New variable to use for the evaluation of the fit
yhat=10.^[polyval(P,Xi)];  
loglog(x,y,'s')
hold on
loglog(10.^Xi,yhat) % Modified plotting. 
xlabel('(mm)')
ylabel('(%)')
grid on
slope = P(1)
txt = ['m=' num2str(P(1))]
text(0.4,40,txt)

From that you might need to adjust the Xi-variable and zoom in the plot.

HTH

[A. Panton]

Thank you very much, but the log10 line gives an error I think it should be like this

Xi = log10(linspace(min(x)/10,max(x)*10,101)); % New variable to use for the evaluation of the fit

[Bjorn Gustavsson]

Yes, I goofed...