Can you clarify what you mean by "max graphs"? Weathermap doesn't draw graphs. If you mean using dsstats targets, can you post the config for one please? They each perform one database query - if anything they should be faster than regular rrdtool targets. That's why I mentioned convert-dsstats.php before.
For the warnings - it's not telling you that the target is incorrect. There's no "validation" to be done in that sense. It's saying that it asked rrdtool to get data from the last few minutes (I think it's 15 by default), and didn't get any data, but NaNs. If you look at that file, has it been updated in the last while? If you look at the graph generated from the data in cacti, does that have a line for the most recent times?
ls -l /opt/lampp/htdocs/cacti/rra/nodeXX_traffic_in_6876.rrd
rrdtool info /opt/lampp/htdocs/cacti/rra/nodeXX_traffic_in_6876.rrd
should tell you when it last received useful data.
(You can get a time breakdown per-map without DEBUG logging, by changing line 220 of lib/poller-common.php to begin warn
instead of debug
, by the way, in 0.97a)