What is the Daily graph?
The daily graph from the trend reports displays the last 24 hours worth of data for a service. As an example(same as the above one) a daily disk usage graph: This is primarily useful for spotting problem trends as they unfold. By itself, it is often hard to see if a pattern is out of the ordinary or not, but alongside some of the other graphs you should be able to see if usage is within the bounds you expect.
