imhist() does not have any possibility of passing in an an axes.
You will need to use the form of imhist() with outputs, and then draw the results. Something like
a = app.a;
app.gray_a = im2gray(a);
[counts, locations] = imhist(app.gray_a);
imshow(app.gray_a, 'Parent', app.UIAxes_2);
hold(app.UIAxes_2, 'on');
bar(locations, counts, 'Parent', app.UIAxes_2);
hold(app.UIAxes_2, 'off');
Notice the lack of global variables. Using global variables in App Designer is most often... misguided...
