Explain which cells are counted for onion service traffic graphs

I can't work out what is being measured on the onion service traffic graph: https://metrics.torproject.org/hidserv-rend-relayed-cells.html

Does it include:

cells sent from rendezvous points to clients?
cells sent from rendezvous points to services?
cells sent to rendezvous points from clients?
cells sent to rendezvous points from services?

Also, the related blog post says: "A related statistic here is "How much of the Tor network is actually hidden service usage?". There are two different ways to answer this question…" https://blog.torproject.org/some-statistics-about-onions

Does the graph try to answer this question? Or is it just measuring rendezvous point traffic without counting the traffic on the relays on rest of the circuit?

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