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?