Never done this myself but here is a previous response from fellow MVP Cindy
Meister. I don't know whether it will help slve the specific merge problem
you have:

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It's sorta possible, but a bit painful.

Assuming you've already got the charts in Excel, then the
solution isn't too bad. Each chart will have a designation;
if you created them on separate sheets, it will be the
sheet name. Otherwise, Excel will have assigned an object
name (or you could change this name via VBA).

Whatever. You need to add a column to the merge data table
and enter the chart name for each record.

Set up the main merge document with everything except the
chart.

In Word, a LINK field will take care of bringing in the
chart. In order to generate the basic field, copy/paste
special + link any chart into Word.

Press Alt+F9 to reveal the field codes (Alt+F9 again to
hide them).

Look at the LINK field code: you should see the chart name
near the end, after the path to the file containing the
charts. Substitute the appropriate merge field for the
chart name.

Without hiding the field codes, or doing anything else,
merge immediately to a NEW document. Ctrl+A, F9, Alt+F9
(select all, update the fields, turn off the field codes).
With any luck, you should now see the charts for each
record.
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Peter Jamieson - Word MVP