Forcing fit to use specific values

I'm trying to fit an equation to a data set. I got a function file which contains the equation lets call it function_eq and i have a main function. Main function is like this :

ops1 = fitoptions('Method','nonlinearleastsquares');

ftyp1 = fittype( 'function_eq(x,lx)' ,'options',ops1,'independent','x','coefficients',{'lx'});

fitted1 = fit(xdata,ydata,ftyp1);

Let's say xdata=[1 2 3 4 5 6] and ydata=[3 5 8 9 10 14]. In this situation main function is trying to pass x values like 0.33,0.66,1.33 to function_eq to solve the fit. But my function_eq is valid only on the exact values of x=1,2,3,4,5,6. I have to force fit algorithm to use only these x values but i couldn't find how to do that.

Note: function_eq calculates a complex numerical integration by given values so another function you recommend may not be suitable but i'm open to new ideas.