I therefore add half the Y-axis range to the actual values before plotting.
Note: these graphs are not all to the same vertical scale!
Later additions have included remote monitoring of the NTP server performance, and more general monitoring of the Raspberry Pi using the standard SNMP functions, with an additional CPU temperature monitoring add-on. Note that good performance is dependant on the GPS unit having a clear view of the sky, particularly the southern part of the sky if you are in the northern hemisphere.
With older GPS receivers this required an outdoor antenna, but more modern units such as those mentioned here may well work indoors providing that some sky is visible, perhaps on the top floor of the building (as I am).
The resulting performance is good, but it will depend on both the loading of the link between me and the ISP, and the general load on the ISP's network and the general Internet.
Offsets are reported to be with about /-5 milliseconds (and therefore off-scale once on the graph below).
The easiest approach with good performance is described here.