[Xymon] Ubuntu Xymon server - local netstat graph issue

I have a 4.3.28 server running on Ubuntu 18.04 server.  The graphs for the local client look OK except for the "TCP statistics" graph, where I get IN packets graphed, but the OUT and RETRANS are all NaN.

netstat graph

Graph for a different host (a CentOS 6 server with a -4.3.10 client) looks fine, all statistics graphed.

I've tried deleting the netstat.rrd for the host, and restarting Xymon server, but doesn't help.  Is there something else I can try?  Thanks in advance.