vedesh - in the pushbutton callback, you would use the handles structure to get the selections from the two dropdown menus. Your code could look something like the following
function pushbutton1_Callback(hObject, eventdata, handles)
% get the selection from the first drop down menu
popup1List = get(handles.popup1,'String'); % get the list of items of popup1
popup1Index = get(handles.popup1,'Value'); % get the index of the selected item of popup1
popup1Value = popup1List{idx}; % get the value of the selected item of popup1
% get the selection from the second drop down menu
popup2List = get(handles.popup2,'String'); % get the list of items of popup2
popup2Index = get(handles.popup2,'Value'); % get the index of the selected item of popup2
popup2Value = popup2List{idx}; % get the value of the selected item of popup2
% note that popup1Value and popup2Value are strings so you may need to convert to numbers is
% applicable
% do whatever work you need to and draw line on axes
