The page to look at is not the graph, but the Workers->Shifts page.
That shows your average hash rate for each shift, which is in no way affected by the shifts before or after it.
Your hash rate was low for your first shift coz you started in the middle of the shift.
The next 9 full shifts you were 4.7 to 4.9THs
Then it dropped to 3.33 and has stayed between 3.1 and 3.5 for the rest (12 shifts so far) - i.e. something went wrong with it (as I said) during shift "8axpj mutsumi"
That's what the numbers mean - the shift also tells you when it started (UTC) and how long it was, so you can tell when it happened.