Fix for #182: Last is always null
Fix for #182: Last is always null

Especially with asynchronous writing to RRD files, plot won't be able to
get the latest data. Instead the graph will end some seconds before. Now,
the "last" value is actually the last value shown in the graph.

vovkasm/facette,iaean/facette,yonglehou/facette,githubert/facette,vovkasm/facette,iaean/facette,yonglehou/facette,hirocaster/facette,iaean/facette,facette/facette,githubert/facette,facette/facette,hirocaster/facette,facette/facette,facette/facette,hirocaster/facette,yonglehou/facette,vovkasm/facette,githubert/facette