OK, I think we're getting closer to figuring this out. From what you and your colleague are saying, it sounds like time is really an important component here. For any given minute, you want to know:
A) how many requests there were (and since each request is represented by a document, this is the same as the number of documents there are in that minute)
B) the average latency value for all of the requests in that minute
Because time is an important component here, I still think you want to use a Date Histogram and then adjust the timepicker to choose the time range you want to visualize. I can't think of any other way to define a range for the x-axis. The original visualization I proposed satisfied B, but not A. We can layer on additional data in this visualization to satisfy A as well. Take a look at the screenshot below, again replacing "machine.ram" with "latency":
On the left axis you'll have the average latency and on the right axis you'll have count. You can see both values for any given minute. You can use the timepicker to specify any time range you want to display in this visualization.
Does this help?
CJ