The error occurs because the scope of the variable S is locally restricted to the function choose_Callback. What I personally do is save these in the handles structure.
Here is what you can do :
function choose_Callback(hObject, eventdata, handles)
% hObject handle to choose (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
switch get(handles.choose,'Value')
case 1
handles.S.A=8.021;
handles.S.B=1936.01;
handles.S.C=258.451;
handles.S.Tmin=18;
handles.S.Tmax=118;
handles.S.T=S.Tmin:((S.Tmax-S.Tmin)/100):S.Tmax;
case 2
handles.S.A=8.021;
handles.S.B=1936.01;
handles.S.C=258.451;
handles.S.Tmin=18;
handles.S.Tmax=118;
handles.S.T=S.Tmin:((S.Tmax-S.Tmin)/100):S.Tmax;
case 3
handles.S.A=7.11714;
handles.S.B=1210.595;
handles.S.C=229.664;
handles.S.Tmin=-13;
handles.S.Tmax=55;
handles.S.T=S.Tmin:((S.Tmax-S.Tmin)/100):S.Tmax;
end
handles.S.P=10.^(handles.S.A-(handles.S.B./(handles.S.C+handles.S.T)));
plot(handles.S.T,handles.S.P)
set(handles.choose, 'UserData', handles.S);
guidata(hObject, handles); %to update the handles structure
And the button callback
function pressureset_SelectionChangedFcn(hObject, eventdata, handles)
% hObject handle to the selected object in pressureset
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
sel_obj = get(handles.pressureset, 'selectedobject');
avial_object = get(handles.pressureset,'children');
if sel_obj==avial_object(1)
set(handles.choose,'value')
elseif sel_obj==avial_object(2)
set(handles.choose,'value', handles.S.P.*(101.325/760))
guidata(hObject, handles); %to update the handles structure
