[mrtg] Math operations

[lucap]

Hi guys

I'm trying to monitor the rate of retransmitted tcp segments and the rate of 
tcp errors.
Following my MRTG (2.16.2) configuration:

WorkDir: /myworkingdir
LoadMIBs: /usr/share/snmp/mibs/TCP-MIB.txt
Refresh: 300
Interval: 5
WriteExpires: Yes
LogFormat: rrdtool

Target[target]: tcpRetransSegs.0&tcpInErrs.0:community at target / 
tcpOutSegs.0&tcpInSegs.0:community at target * 100
Directory[target]: target
MaxBytes[target]: 100
kMG[target]: ,,
Unscaled[target]: dwmy
Options[target]: growright, nopercent
ShortLegend[target]: %
YLegend[target]: TCP segments (%)
Legend1[target]: Retransmitted segments
Legend2[target]: Received segments with errors
LegendI[target]: Out
LegendO[target]: In
Title[target]: TCP segments
PageTop[target]: <H1>TCP segments</H1>

I have no errors, but data in the RRDTool file are always 0 (zero):

rrdtool fetch test.rrd AVERAGE
                      ds0                          ds1
.....................................................................................
1432024500: 0.0000000000e+00 0.0000000000e+00
1432024800: 0.0000000000e+00 0.0000000000e+00
1432025100: 0.0000000000e+00 0.0000000000e+00
1432025400: -nan -nan

RRDTool version is 1.3.8.

Thank you in advance for any suggestions.

Best Regards
Luca