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xan

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Posts: 44
Reply with quote  #1 
Is there a general term that encompassing both dodging and jittering? Or is one a subset of the other?

I had been treating jitter as a kind of dodging, but below is an excerpt from Wilkinson's Grammar of Graphics suggesting the terms are peers. My current term is that dodge and jitter are kinds of "offsetting," but I'd rather use an accepted term if there is one.

dodgejitter.png 

sfew

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Reply with quote  #2 
Xan,

Apart from Leland's book, I've never encountered the term "dodge." It appears that Leland would describe the form of over-plotting avoidance that is used in a dot histogram or a wheat plot as dodging. He also seems to call the side-by-side positions of clustered bars a form of dodging. Technically, jittering involves random repositioning of objects, but I don't restrict the term in this manner. In my opinion, random repositioning never produces the most useful results. I, and many others, use the term jitter more generically to refer to all forms of repositioning that are used to avoid over-plotting when multiple data encoding objects share the same quantitative value and are therefore share the same position in the graph. Leland's graphical grammar treats all occasions when objects share the same position along a scale, either quantitative or categorical, as "collisions" in need of modification. For example, when multiple values are associated with each item along a categorical scale in a bar graph (e.g., a actual expense and a budget expense value), the collision may be avoided by either placing them side by side (i.e., clustered bars) or by stacking them (i.e., stacked bars). In contrast, I think of objects that are associated with the same categorical item as different from objects that share the same quantitative value. Placing bars side by side doesn't change alter their quantitative values, but jittering objects does. For this reason, I think of them as different.

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Stephen Few
xan

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Reply with quote  #3 
Thanks, Steve. Very helpful.

Incidentally, "dodge" is used by R's ggplot2 and probably other packages that follow the Grammar of Graphics architecture.

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