# How to best display forecast deviation?

I have to represent some forecasting deviations into a chart. The problem is that I can’t find the proper way to do it. I don't know which approach to use in order fit with the data type I'm working with. The biggest is the traffic the more accurate the forecasts is. The following table represents the forecast deviation and traffic consumption for 5 users. The biggest deviation was made by the first user and not by the second user. How to represent that in a chart?

Deviation (%)      2.6     22.8       2.5     15.9      2.3
Traffic (GB)      17.7     0.08      15.2      0.1     13.3


I can do the calculus of deviation*traffic/100 and to put those in a chart but maybe there is a better solution. Maybe I have to use other logic?

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So the Deviation is geometric, and you want to visualize it in arithmetic space? I see no problem with your obvious solution of multiplying deviation by traffic. You could also plot the Traffic values with error bars. –  shabbychef Jun 22 '12 at 16:42

After looking at your sample data (and assuming its fairly representative of your actual data), the thing that jumped out was the relatively low actual traffic value, regardless of forecast deviation. So, you could consider two charts to show your data:

1. Chart of actual traffic
2. Chart of actual deviation (not percentage).

Using a small-multiple approach with the same scale, you can show the actual traffic with the calculated deviation from forecast, and get a sense of their relative impacts. This helps emphasize that regardless of the percentage deviation, the relative impact is small to the overall traffic usage.

If you really want to make that impact, you could include a total traffic/deviation line which would really de-emphasize the large deviation/small traffic entries. Obviously, you lose some finer detail (not that there was much to begin with), but provide a better overall picture.

EDIT: Here's a copy of the bottom chart with Excel's normal gridlines turned on and the chart areas shaded (left with transparency and right without). The Excel Bar Charts have everything default stripped out and then are re-designed with the minimum structure necessary to convey the info, then they're lined up with the appropriate spreadsheet rows.

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nice answer; how did you implement these figures? –  Abe Jul 27 '12 at 15:49
Thanks, they're just Excel-but with absolutely no defaults left over. –  dav Jul 27 '12 at 16:04