A histogram of a continuous variable is *not* primarily a graph of
counts within a range, but of densities; that is, an approximation to a
density function. Therefore, it is admissible to join or subdivide bars
provided that the area of the bar, not the height, represents the joint
count.
When the ranges are equal, one can label the graph on one vertical edge
with densities and on the other with counts.
There might be situations in which the counts were truly more imortant
than the density - for instance, "pass-fail" statistics based on
numerical grades, or "too narrow-acceptable-too wide" for ball
bearings. I would argue that in such a situation the count
classifications were essentially categorical and should be represented
by narrow, separated, bars to make this clear.
-Robert Dawson
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