Im pretty sure it can’t be cumulative since you have peaks and valleys in the plot. I think it’s just showing the visits per day during a period that goes from 16th May to 11th June. They’re just saying “over time” since the “time” is the x axis. The y axis is simply the number visits on that day (I think)
Daily traffic visits to the top two #Kbin and #Lemmy servers over the last 28 days, up until June 11th.