[Xymon] CPU trend graph NaN - log shows RRD Error Updating , unknown DS name 'mem_swpd'

[Betsy Schwartz]

I'm seeing a few RHEL6 servers with blank CPU trend graphs .
the rrd files just contain NaN, sample line:

 /usr/bin/rrdtool dump  /usr/local/xymon/data/rrd/
ext-dns1.example.com/vmstat.rrd

                       <!-- 2015-07-17 20:00:00 EDT / 1437177600 -->
<row><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN
</v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN
</v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v><v> NaN </v></row>

rrd-data.log has errors of the form /rrd-data.log:2015-07-18
14:08:42.443123 RRD error updating /usr/local/xymon/data/rrd/
ext-dns1.example.com/vmstat.rrd from 10.100.5.42: unknown DS name 'mem_swpd'

So,why do only a few of my servers have this error?
What do I need to do to make the CPU trends graphs work? Do I add mem_swpd
in xymonservers.cfg and hobbitgraph.cfg and graphs.cfg, and can I do that
without breaking the other hundreds of servers?

Does anyone have the correct invocation for this?
Thank you!
(I am also assuming I'll have to blow away the old rrd file to get this to
work...?)
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