Fix stacked bar and utilisation style plots handling of (temporarily) missing data, particularly the stacked bar case.  Resolves an issue which prevented the Memory utilisation view from displaying any data at all.
Fix stacked bar and utilisation style plots handling of (temporarily)
missing data, particularly the stacked bar case.  Resolves an issue
which prevented the Memory utilisation view from displaying any data
at all.

aeg-aeg/pcpfans,tjanez/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,wuliming/pcp,edwardt/pcp,mbaldessari/pcp,wuliming/pcp,prasincs/pcp,mbaldessari/pcp,prasincs/pcp,adfernandes/pcp,tjanez/pcp,adfernandes/pcp,prasincs/pcp,wuliming/pcp,andyvand/cygpcpfans,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,tjanez/pcp,wuliming/pcp,prasincs/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,edwardt/pcp,mbaldessari/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,edwardt/pcp,mbaldessari/pcp,tjanez/pcp,mbaldessari/pcp,wuliming/pcp,andyvand/cygpcpfans,edwardt/pcp,adfernandes/pcp,adfernandes/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,edwardt/pcp,adfernandes/pcp,edwardt/pcp,prasincs/pcp,prasincs/pcp,tjanez/pcp,adfernandes/pcp,edwardt/pcp,aeg-aeg/pcpfans,tjanez/pcp,edwardt/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp