[Xymon] "ports" RRD graph is often flaky.

I see several graphs like this in xymon (4.3.23), where at least one of the port graphs (configured in analysis.cfg) has lots of data points missing:

https://www.dropbox.com/s/l483lilzh5n610k/flaky-xymon-port-graph.png?dl=0

Any idea how I can fix this? As you can see, the red line for http has data for the entire graph, while the blue line for ajp is very spotty. I just added the last two monitors less than an hour ago, so I don't have history on those yet.

This is not unique to this host. As far as I can tell, the port listings coming back from the client are always good, even when the graph has no line.

Thanks, Shawn