In interface.lua and ping.lua in /usr/lib/lua/luci/statistics/rrdtool/definitions/, I found I had to use %di instead of %pi in order to get any data displayed.
This is fixed since Turris OS 3.10.1
https://gitlab.labs.nic.cz/turris/openwrt/issues/189
as with many other packages are now terribly out of date.
We know about it and this state is just temporary.
You can request to update packages, which you like in this thread: Requesting new packages / new versions - #44 by Pepe - Testing branches - Turris forum
I don’t want to promise you that we’ll update all of them, but I’ll look at them if you want.
I think the rest of the issues I had were with things like support files, /usr/bin/stat-genconfig and
/usr/lib/lua/luci/controller/luci_statistics/luci_statistics.lua, which were missing pieces for creating the configs for the graphs and space for the graphs and config areas for the graphs in the web interface.
Will try to reproduce.