Image_Graph: overlapping bar chart

Hi,

I wonder if the following could be accomplished, and if so, how:

Currently I've got a Graph showing 3 bars, side by side, while I'd
like to have them overlap:

^
|               +---+
|               |   |
|               |   |
|               |   |
|        +---+  |   |
|        |   |  |   |
|        |   |  |   |
| +---+  |   |  |   |
| |   |  |   |  |   |
+--------------------->



^
|            +------+
|            |      |
|            |      |
|            |      |
|        +-------+  |
|        |       |  |
|        |       |  |
| +--------+     |  |
| |        |     |  |
+--------------------->

Any advices greatly appreciated!

Thanks,
--
Michael - < mike(@)php.net >