Drawing with mouse motion in a GUI-image

Hi, I want to draw a line in an image displayed in a GUI. Matlab has a good example which works great for an individual figure:

http://www.mathworks.com/help/matlab/ref/line.html

However, while running it from a callback function in the GUI, I am having issues:

1. Defining handle for the image-axis is not allowed by WindowButtonDownFcn.
2. I can use the handle of the GUI-figure (which is an alternative of the point above), but how do I extract the co-ordinates for later use?

Example code (Matlab)

function draw_lines
% Click the left mouse button to define a point
% Drag the mouse to draw a line to the next point and
% left click again
% Right click the mouse to stop drawing

% 
figure('WindowButtonDownFcn',@wbdcb)
ah = axes('DrawMode','fast');
axis ([1 10 1 10])
   function wbdcb(src,evnt)
      if strcmp(get(src,'SelectionType'),'normal')       
         [x,y,str] = disp_point(ah);
         hl = line('XData',x,'YData',y,'Marker','.');
         text(x,y,str,'VerticalAlignment','bottom');drawnow
         set(src,'WindowButtonMotionFcn',@wbmcb)
      elseif strcmp(get(src,'SelectionType'),'alt')
         set(src,'WindowButtonMotionFcn','')
         [x,y,str] = disp_point(ah);
         text(x,y,str,'VerticalAlignment','bottom');drawnow
      end
      function wbmcb(src,evnt)
         [xn,yn,str] = disp_point(ah);
         xdat = [x,xn];
         ydat = [y,yn];
         set(hl,'XData',xdat,'YData',ydat);
      end  
   end
   function [x,y,str] = disp_point(ah)
      cp = get(ah,'CurrentPoint');  
      x = cp(1,1);y = cp(1,2);
      str = ['(',num2str(x,'%0.3g'),', ',num2str(y,'%0.3g'),')'];    
   end
end

[Image Analyst]

Are you using GUIDE? Or are you trying to do it the hard way and define all the properties/settings and callbacks yourself?

And what's in the axes control? If there is something like an image you'll have to set a callback for the image since it lies on top of the axes, or else set the hittest for the image to be off.

[ImageProcNaive]

Yes, using GUIDE. Also, there will be an image displayed in the axes. I have to draw a line along an object displayed in the image. I will later measure the length of the line; but it's important to have the visual of a line.