The problem lies with PASTING the Excel chart into PM. You should never paste any graphics into PM, especially if it is to be converted to a PDF. Far better to use File>Place.
However, to get Excel charts into PM, I use the following method:
(1) save the Excel chart as a PDF (use Distiller as your printer).
(2) use File>Place to get the chart into PM.
If your version of PM will not accept PDFs, then there is an import filter available on the adobe website (I will see if I can find a link).
You will encounter another problem if the final PM file is to be commercially printed. The Excel PDF will be in RGB mode and needs to be converted to CMYK. This can be done by OPENING the PDF in Illustrator. You may need to ungroup the chart first but it wil be fully editable. You can click on each coloured area and convert to CMYK (or even a spot colour if that's what you want). Add anything else you want in AI and save it either as an EPS (which will get around the lack of the import filter) or as a PDF, either of which you should use File>Place to get it into PM. Note that EPS graphics will print well only on a postscript printer.