Bar History

Bar graphs are commonly employed to show level-based PVs. One possible way of integrating trend information into this kind of display that we explored was overlaying the value of that variable at various points in time in the past. The effect is similar to a constantly filling measuring cup where the level is marked off at given time intervals.

The primary value of this display beyond just raw history is that since the interval between points is constant, the difference between the sections of the bar graph would help to indicate the speed of the process. One of the clear limitations to this technique is that the process would need to behave in a generally monotonic fashion for the sizes to be comparable.

An alternate approach for displaying history information would be to have the history bars beside each other.

The logical extension of this would be to overlay a line graph plotting the history. This final technique would much more accurately communicate history for processes that rapidly oscillate between increasing and decreasing. The area between the current value and the historical value could be colored to highlight the change.