I had problems trying to delete a chart (graph) in Calc. I kept getting the error message "This function cannot be completed with the selected objects". I followed the instructions from several different sites, but nothing worked. What finally worked for me was to perform the following steps:
close and save the spreadsheet
open the spreadsheet
select the object (chart, graph, etc)
press delete key
I do not know why, but doing anything else will display the error message and the chart will remain.
I suggest playing with charts and graphs in a disposable copy of the original spreadsheet.