That actually depends on what is being plotted. Some are flagged as “gauge: moving average only counts days with a measurement” (e.g. Resting HR). Others (e.g. Training load) include all days.
Is it possible to change that for a non-custom field? Let’s say weight as I might not weight myself everyday and don’t see why 0 should affect the moving average. Actually, is it possible to change the weight so that it is flagged as “gauge: moving average only counts days with a measurement?