I am not sure what you want.
Try this:
fun = @(x) (x.^2 + 2)
q = integral(fun,-1,2)
xv = linspace(-2,3);
xseg = xv(xv>=-1 & xv<=2);
figure
plot(xv, fun(xv))
hold on
patch([xseg fliplr(xseg)], [fun(xseg) zeros(size(xseg))], 'b', 'FaceAlpha',0.25)
hold off
text(mean(xseg), q*0.1, sprintf('$\\int_{%d}^{%d}x^2+2 dx = %d$',-1,2,q), 'Horiz','center', 'Vert','middle', 'Interpreter','latex')
.
