A histogram window for the cpi example is shown in Figure 5 .
The histogram view is, like the timeline view, highly interactive. The panel at the top left describes the total histogram data, such as the range of values and the number of values. The top right panel describes the data that is presented in the histogram window itself. The number of bins for the histogram can be varied with the slider control, which dynamically changes the shape of the histogram. The histogram window can be zoomed and scrolled to select a particular set of state instances, and the ``Blink states'' button causes those states to blink on and off in the timeline view so that they can be easily located. We have found this to be an effective method for locating specific state instances even in a very busy display.
The most novel feature of this display is the method used to find states that last ``too long.'' It makes the (not always true) assumption that each instance of a particular state should last approximately the same length of time, and therefore the state durations are liable to have a normal distribution. (This can be viewed as an application of the Central Limit Theorem of statistics.) Under this assumption, one can calculate the high and low cutoff that would identify particular percentages of the states durations. If the states truly last nearly all the same length of time, then this method of identifying the top 1%, say, of the states will find none at all. Thus we can attempt to find states with ``anomalous'' lengths, not just the longest and shortest durations. Clicking on the various percentage buttons selects a particular set of state instances, which easily can be identified in the timeline display when the user clicks on ``Blink states''.
This technique is admittedly crude. One of our research tasks over the next few years is to develop a sophisticated set of techniques for drawing the user's attention to that part of the computation where tuning can bring performance benefits.
The ``Resize to fit'' button raises the height of the histogram bars if the selection is such that they have become inconveniently short. The ``Selected Regions'' button allows one to selectively turn off the blinking of selected sets of state durations, the ``Print'' button prints the histogram window in Postscript, and ``Close'' exits.