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
