To edit a chart as individual graphic elements, you must first Copy or Cut it, then Paste and Ungroup. Note: Ungrouping a chart breaks the link to the spreadsheet data so it will not update with changes.
Explanation:
Only a PICT or WMF file can be ungrouped. When a chart is initially created, it is not a PICT/WMF since it is still tied to the spreadsheet that originally created it. This is done to allow for the automatic updating capabilities of a chart.
Therefore, to be able to ungroup a chart to edit its individual elements, it first must be copied and pasted to transform it into a PICT.