Why I can draw lines on the Axes but can't draw lines anymore after imshow a picture on Axes?

Hi, I want to build a gui to draw lines on an image. I can draw lines before load a image on Axes. But after I imshow the image, which I want to draw lines on, on Axes, I can't draw lines anymore. And I also tracked the data. The data can be stored before imshow an image. But nothing happened after load the image. Here is part of my code for open a file and draw a line. Moreover, I have already defined all the values at very beginning. Hope it can be solved. Thanks in advance.

       function open_ClickedCallback(hObject, eventdata, handles)
      % hObject    handle to open (see GCBO)
      % eventdata  reserved - to be defined in a future version of MATLAB
      % handles    structure with handles and user data (see GUIDATA)
      [filename,pathname]=uigetfile('*.*');
      handles.img=imread([pathname filename]);
      handles.imggray=rgb2gray(handles.img);
      imshow(handles.imggray);
      guidata(hObject,handles);      function drawlines_Callback( hObject, eventdata,handles)
      % hObject    handle to drawlines (see GCBO)
      % eventdata  reserved - to be defined in a future version of MATLAB
      % handles    structure with handles and user data (see GUIDATA)
      set(handles.imgwindow,'ButtonDownFcn',{@start_pencil,handles})      guidata(hObject,handles);      function start_pencil(hObject,eventdata,handles)      coords=get(gca,'CurrentPoint'); %since this is the axes callback, src=gca
      x=coords(1,1,1);
      y=coords(1,2,1);      r=line(x, y, 'color', 'g', 'LineWidth', 2, 'hittest', 'off'); %turning     hittset off allows you to draw new lines that start on top of an existing line.      handles.r = [handles.r r];      set(gcf,'windowbuttonmotionfcn',{@continue_pencil,r,handles})
      set(gcf,'windowbuttonupfcn',{@done_pencil,handles})      guidata(hObject,handles);      function continue_pencil(hObject,eventdata,r,handles)
      %Note: src is now the figure handle, not the axes, so we need to use gca.
      coords=get(gca,'currentpoint'); %this updates every time i move the mouse
      x=coords(1,1,1);
      y=coords(1,2,1);      %get the line's existing coordinates and append the new ones.
      lastx=get(r,'xdata');  
      lasty=get(r,'ydata');      newx=[lastx x];
      newy=[lasty y];      global C;
      C=[newx;newy];      guidata(hObject, handles);
      set(r,'xdata',newx,'ydata',newy);      function done_pencil(hObject,eventdata,handles)
      %all this function does is turn the motion function off 
      set(gcf,'windowbuttonmotionfcn','')
      set(handles.imgwindow,'ButtonDownFcn','')
      set(gcf,'windowbuttonupfcn','')
      guidata(hObject, handles);

Answer 1

[Rui]

The problem solved. I just add a new handle to the imshow and set its buttondownfcn. Thanks, anyway, to everyone who concerned my problem.