Hi have problem when doing surface fitting the surface that i obtained has the x and y axis switched. The code is:

 ft = fittype( 'poly33' );
    opts = fitoptions( ft );
    opts.Weights = zeros(1,0);
    coeffnames(ft)  %mostra os coeficientes
    
    formula(ft)   %mostra a equação final
      
    
    c = fit( [xi, yi], zi, ft, opts )
    
    coef = coeffvalues(c)
    
    figure( 'Name', 'Surface fitting' );
    h=plot( c, [xi, yi], zi );

And when i going to cheek the surface it apperes that x and y are exchanged.

figure
    step_x=(2*(a+a_i*1/4+a/10))/50;
    step_y=0.007/50;
    
    for i=1:50+1
        x_v(i,1)=-a-a_i*1/4-a/10+step_x*(i-1);
        y_v(i,1)=step_y*(i-1);
    end

    clear x y
    for i=1:50+1
        for j=1:50+1
            x=x_v(i,1);
            y=y_v(j,1);
    Z(i,j)=coef(1,1) + coef(1,2)*x + coef(1,3)*y + coef(1,4)*x^2 + coef(1,5)*x*y + coef(1,6)*y^2 + ...
        coef(1,7)*x^3 + coef(1,8)*y*x^2  + coef(1,9)*x*y^2 + coef(1,10)*y^3;
        end
    end
    
    surf(x_v(:,1),y_v(:,1),Z(:,:))

Thanks!