Graph question

Given this graph showing the user ramping up with "setUp(scn.inject(rampUsersPerSec(1) to(10) during (10 minutes))).maxDuration(12 minutes).protocols(httpConf)"

After some time the response time graph disappears:

Why? In addition, this is expected right? that response times correlates in an increasing manner with the number over users I am ramping up with?

Is there another plausible explanation why my users is having a lot more responses per second after they are all ramped up (the user graph is straight), than that the application can’t deal with so many concurrent users? The pattern is very clear and I figured it might be my misunderstanding or that I understand the graph wrong?

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