r squared comparing sine waves

I have a dataset to which I have asked matlab to fit a sine wave. I want to find the r^2 value to see how well the sine curve I have generated fits the data.

Is there a simple way of doing this?

x1 = time';
y = Qe_mean;
yu = max(y);
yl = min(y);
yr = (yu-yl);                
yz = y-yu+(yr/2);
zx = time(yz .* circshift(yz,[0 1]) <= 0); 
P = period;                  
ym = mean(Qe_mean);                
fit = @(b,x)  b(1).*(sin(2*pi*x./P + 2*pi/b(2))) + b(3);      
fcn = @(b) sum((fit(b,x1) - y).^2);                              
s = fminsearch(fcn, [yr;  -1;  ym])                                    
xp = linspace(min(x1),max(x1),P);

figure()
plot(xp,fit(s,xp), 'b')
hold on
plot(time,y,'k','Linewidth',1)