Matlab - hacer una interfaz grafica que permita aproximar el area bajo la curva de una funcion

function varargout = area_bajo_la_curva(varargin)

gui_Singleton = 1;

gui_State = struct('gui_Name',       mfilename, ...

                   'gui_Singleton',  gui_Singleton, ...

                   'gui_OpeningFcn', @area_bajo_la_curva_OpeningFcn, ...

                   'gui_OutputFcn',  @area_bajo_la_curva_OutputFcn, ...

                   'gui_LayoutFcn',  [] , ...

                   'gui_Callback',   []);

if nargin && ischar(varargin{1})

    gui_State.gui_Callback = str2func(varargin{1});

end

if nargout

    [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});

else

    gui_mainfcn(gui_State, varargin{:});

end

function area_bajo_la_curva_OpeningFcn(hObject, eventdata, handles, varargin)

handles.output = hObject;

guidata(hObject, handles);

function varargout = area_bajo_la_curva_OutputFcn(hObject, eventdata, handles)

varargout{1} = handles.output;

function edit1_Callback(hObject, eventdata, handles)

handles.f=get(hObject,'String');

guidata(hObject, handles);

function edit2_Callback(hObject, eventdata, handles)

handles.a=str2double(get(hObject,'String'));

guidata(hObject, handles);

function edit3_Callback(hObject, eventdata, handles)

handles.b=str2double(get(hObject,'String'));

guidata(hObject, handles);

function edit4_Callback(hObject, eventdata, handles)

handles.n=str2double(get(hObject,'String'));

guidata(hObject, handles);

function pushbutton1_Callback(hObject, eventdata, handles)

f=handles.f;

a=handles.a;

b=handles.b;

n=handles.n;

delta=(b-a)/n;

ezplot(f,[a,b]);

hold on

for i=1:n

    xi=a+(i)*delta;

    altura1=subs(f,xi);

    altura2=subs(f,xi-delta);

    if altura1==0

        altura1=0.001;

    end

    if altura2==0

        altura2=0.001;

    end

    if altura1<altura2

        altura1

        altura2

       pause(1)

       rectangle('Position',[xi-delta 0 delta altura1],'FaceColor','B');

       ezplot(f,[a,b]);

        pause(1)

        rectangle('Position',[xi-delta 0 delta altura2],'FaceColor','R');

        ezplot(f,[a,b]);

    end

 end

hold off