Graphics Programs Reference
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they handle the ball more often, or do they handle the ball more often because
they are good at scoring points and the coach runs more plays for such players?
Warning: Don't mix up causal and correlative
relationships. They look the same when you
visualize them, but the former is more difficult
to prove than the latter.
Figure 4-39 shows two ways to incorporate a third variable in
a scatter plot. The symbol plot on the left should look famil-
iar because it was used with spatial data on a geographic
scale earlier in the chapter. The area of a circle represents
assists per game. The scatter plot on the right uses color
instead of area to show the same thing. The darker the
shade, the more assists per game.
The hope is that you'll see larger circles or darker shades clustered in an area
of the scatter plot. In Figure 4-40, you see assist leaders closer toward the right
corner of higher usage percentage and points, but there's high variability and
there isn't a clear trend. There are players with a lot of assists per game who
don't score that many points, and there are others who score a lot of points,
have high usage percentage, and a lot of assists. It is clear however that those
who don't score many points and have lower usage percentage typically don't
have many assists either.
You can also double up on encodings, using both size and color to represent
a third variable, as shown in Figure 4-40. The redundant visual cues help
reinforce what might be more of a challenge to see with just one visual cue.
FIGUREĀ 4-39 Symbol plot on the left and colored scatter plot on the right
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