As far as I know you can't get the charts using standard MailMerge fields.
You /might/ be able to get the charts by using a LINK type field in certain
circumstances, e.g. if you have a field rownum containing the row number of
the row in the Excel sheet, a nested field such as

{ LINK Excel.Sheet.8 "C:\\myexcelsheets\\mysheet.xls"
"Sheet1![mysheet.xls]Sheet1 Chart { MERGEFIELD rownum }" \a \p }

ALthough I can make this work here in a very simple scenario,
a. I have to output to a new document, then select it (e.g. ctrl-A) and
press F9 to re-execute all the LINK fields
b. I wouldn't want to rely on it.

Peter Jamieson