Time units on the y-axis

[aalpern]

Now that we have the timerstat dashboard item, re-using the humanize-duration code to render the y-axis in humanized time format (using the compact "language" option) would be useful for lag counters.

Obviously applies to the flot renderer only.

[aalpern]

In fact, adding the ability to specify the y-axis label format in general would be good (see the confusion in #485, for example)