Here the error when I run mrtg from command line..
/usr/bin/rateup: error while loading shared libraries: libpng15.so.15: cannot open shared object file: No such file or directory
2013-05-27 19:05:31: ERROR: Skipping webupdates because rateup did not return anything sensible
2013-05-27 19:05:31: WARNING: rateup died from Signal 0
with Exit Value 127 when doing router '192.168.2.1_1'
Signal was 0, Returncode was 127
[root@alarmpi mrtg]# pacman -Ss mrtg
community/mrtg 2.17.4-1.1 [installed]
Multi Router Traffic Grapher.
[root@alarmpi mrtg]# pacman -Ss libpng
extra/libpng 1.6.2-3 [installed]
A collection of routines used to create PNG format graphics files
community/libpng12 1.2.50-2
A collection of routines used to create PNG format graphics files