How to plot a function with GUI input data?

Hi, I'm new to MATLAB and GUI building and I have problems to plot a function which is given by text edit input.

Here is my code so far.

function pb_plot_Callback(hObject, eventdata, handles)

f_x= get(handles.ed1,'String');
display (f_x);                  %f_x is a String i.e. 'x^2 -2'

x1= get(handles.ed2,'String');
x= str2num(x1);
display (x);                    % x= -2:0.1:2

f_x= [1^2 0 -2];               %How do I convert f_x = 'x^2 -2' into this?
%x= -2:0.1:2
y= polyval(f_x,x);
plot(x,y);

%symbolic solution doesnt work at all
%syms a b
%a= f_x;
%b= x;
%c= subs(a);
%plot(b,c);

So as you can see for polyval I need to convert f_x to a matrix wothout variables.

The symbolic solution results in this error:

??? Subscript indices must either be real positive integers or
logicals.

Error in ==> sym.subs>getNames at 248
    s(end) = [];

Error in ==> sym.subs>tryFunctionHandle at 190
xvarnames = getNames(X);

Error in ==> sym.subs>mupadsubs at 137
[G,worked] = tryFunctionHandle(F,X,Y);

Error in ==> sym.subs at 125
G = mupadsubs(F,X,Y);

Error in ==> subs at 60
r = subs(sym(f),varargin{:});

Error in ==> mathsuite>pb_plot_Callback at 153
c= subs(a);

Error in ==> gui_mainfcn at 96
        feval(varargin{:});

Error in ==> mathsuite at 42
    gui_mainfcn(gui_State, varargin{:});

Error in ==>
@(hObject,eventdata)mathsuite('pb_plot_Callback',hObject,eventdata,guidata(hObject))

??? Error while evaluating uicontrol Callback

so I think symbolic doesn't work at all in GUI callbacks.

I hope you can help me. Thanks in advance.

[Oliver Ferenczi]

Did you figure it out?