pmwebd graphite: cairo rendering: fix time axis
pmwebd graphite: cairo rendering: fix time axis

- timestamp formatting now more clever
- truncation better avoided when scaling from integer times to fp coordinates

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