How super-impose two graphs with independent datasets in a document
The following procedure describes how to super-impose two graphs on top of each other. Each graph can have its own dataset.
1. Add two datasets to a document
2. Add one graph based on one dataset as Bar graph and add another Line graph based on the other dataset.
3. Put one graph onto the other one to overlap, as shown below:

4. Open the above graph Properties and formatting editor, change the fill color to "No Fill"
5. Switch to Flash mode, you should see two graphs overlapped, as shown below:
