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This question is about data visualization, but ultimately it deals with usability and user expectations, so I felt it was appropriate to post it here.

Imagine you are doing a comparison over time between two entities, one entity (or series) represents 'you' and another time series that represents some benchmark entity. The user's analysis question is to try and get a clear picture of how 'you' compares to the benchmark entity over time. In other words, the delta between the two series over time. To do this I opted to flatten one of the series (turn it into a baseline) and make the y-axis show 'delta from baseline'. My question is, which of the two series should be the baseline?

Here is a concrete example:

Here is the basic visual (two series, with the difference highlighted in blue): enter image description here

--- Option A - baseline is 'you' series: enter image description here

--- Option B - baseline is 'benchmark' series: enter image description here

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I'd go with the Benchmark as the constant (Option B). Thinking as a user, I feel this type of visualisation allows me to better comprehend what the data is telling me. Assuming the benchmark is what I'm aiming for, the "You" data points allow me to see how my results pair up. – Daniel Meade Nov 5 '12 at 23:30
@DanielMeade: That's the thing, the benchmark data might NOT be what you are aiming for, it's just a comparison point. You might be working hard to avoid being like the benchmark. – M.A.X Nov 5 '12 at 23:49
Even so, to me it still seems the best fit. I can only see my own results as a constant, and the benchmark floating around it, to be confusing. – Daniel Meade Nov 5 '12 at 23:52
up vote 3 down vote accepted

Definitely Option B, because users want to see how they are doing relative to some constant. It's a self thing.

The variance of the line for 'You' is better perceived and interpreted as being related to you doing the activity, rather than you being the constant and the world fluctuating about you.

It's like asking whether you are (stuck at) the center of the universe and everything revolves around you, or whether the center is elsewhere and you (perceive yourself to) have the control to move around.

It doesn't matter if the baseline is a comparator to reach or to avoid, it's still a relative sense of self.

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