Image Graph - delta display

Hi! I have another question about Image Graph. Can somebody explain a little the inner logic of a graph. I can't figure out how exactly work how an Image_Graph_Elements know where it should be on the Graph. It is not clear how functions _reset, _setCoords, _updateCoords work, I think this function has the responsibility to set the right, left, bottom, top values correctly. Now Plot works like this:

    ^
    |...............................................
    |...............................................

5 -...............................................

    |...............................................
    |...............................................

4 -...............................................

    |...............................................
    |..................................O.....O......

3 -......................O.....O..................

    |...............................................
    |...............................................

2 -..........O.....O..............................

    |...............................................
    |...............................................

1 -....O..........................................

    |...............................................
    |...............................................
    -----|-----|-----|-----|-----|-----|-----|------>
         1     2     3     4     5     6     7

But I have to generate Graphs like this:

    ^
    |.............................................
    |.............................................

5 -.............................................

    |.............................................
    |.............................................

4 -.............................................

    |.............................................
    |................................O.....O......

3 -....................O.....O..................

    |.............................................
    |.............................................

2 -........O.....O..............................

    |.............................................
    |.............................................

1 -..O..........................................

    |.............................................
    |.............................................
    ------|-----|-----|-----|-----|-----|-----|---->
       1     2     3     4     5     6     7

I think Image Graph has a lot of cool feature that's why for me it would be the prefered way to creating graphs like this to extend it. But now I'm not sure which could be the best way to it, is it enough creating a new Plot type, and a new Axis type, or this is a little more defficult? It would be great if the creator of this package could give me some hints about this topcis, I saw erlier he has some post on the list. THX in advance, Felhő