i just updated my ZBT WE1326 to latest, stable 19.07.0. I have installed luci-app-statistics. All seems to be fine except memory module. It is just not updating the graph since i did the update.
Check the possible config options and toggle them.
New config items were introduced rather recently and your old config probably does not yet contain them. You probably need to add this: ValuesAbsolute '1'