PVE rrd data is stored in /var/lib/rrdcached/db/
. If you remove all the directories (pve2-node, pve2-storage, pve2-vm) within this directory, all statistics will be reset, and new directories will be created upon the next poll.
Note that this is part of the cluster file system, so contains the statistics for each node in a cluster. If you instead wanted to, for example, only reset node specific statistics for a single node, you could remove: /var/lib/rrdcached/db/pve2-node/<hostname>