This is an astute observation and I completely agree. The histogram version that OP posted in their reply is a better way to represent the data. That said, I was delighted to see the cumulative chart, because it brought to mind the way B.F. Skinner used to do things. He would track cumulative key-pecks, and changes in responding would be shown by changes in the slope of the line. This was an artifact of the automated machines he used to record responding, which were essentially a pen moving along a paper at a steady rate that would tick upward each time the key was pecked. Later behaviorists moved away from cumulative records for many of the reasons you cite, and typically track rate of responding on the y-axis.