Why you want to use fplot? You can striaght away use polyval.
load=[0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0];%measured data
efficiency=[0.274766154 0.567610421 0.700286086 0.766121699 0.831135824 0.848797812...
0.868255235 0.881644769 0.892014 0.898709 0.9018];%measured data
g=5;%polynomial degree
p=polyfit(load,efficiency,g);%coefficient calculation
x = linspace(0,0.9,20) ;
y = polyval(p,x,'*r') ;
f=@(x) p(1)*(x)^5+p(2)*(x)^4+p(3)*(x)^3+p(4)*(x)^2+p(5)*(x)+p(6); %polynomial function
figure1=figure('color','w','Position',[1,41,1000,500],'Renderer', 'Painters');
fplot(f,[0 0.9],'linewidth', 1.5,'color',[0.8500, 0.3250, 0.0980]);%plot
hold on
plot(x,y,'sb')
If you are supposed to use polynomial then you can achieve that using poly2sym. Read about this function.
