Why are histograms often filled?

Many histogram functions part of scientific analysis software has the default configuration to fill the histogram. Examples are Matlab and Matplotlib. On the other hand, the default for line plots is to not fill the area below the line. Both behaviours can be altered.

One disadvantage of filling is that it makes it more difficult to visualise different quantities in the same axes, unless it is natural to stack them. What is the reason that it is usual to fill the area under a histogram?

Below is the same data with histograms shown in three different ways using Matplotlib:

A traditional bar-style histogram (hist(X, 50, histtype='bar', edgecolor='none')):

A step-style histogram (hist(X, 50, histtype='step')):

And finally, a step-fill histogram with α=0.2 (hist(X, 50, histtype="stepfilled", alpha=0.2)):

• I don't know whether we here at CV are the best people to ask for why something is visualized in a particular way, although we likely can supply no end of opinion. If you don't get a good answer here, you may want to consider flagging your question and requesting migration to UX.SE, where they in fact do have questions on histograms. – Stephan Kolassa Dec 4 '15 at 16:06
• @StephanKolassa I was in doubt between here and Graphic Design, didn't think of UX. Despite the question being why, I believe it should be answerable based on research on how information is perceived, though. – gerrit Dec 4 '15 at 16:09
• "based on research on how information is perceived" - which I'd expect the nice folks at UX.SE to be more familiar with than us. We will see. – Stephan Kolassa Dec 4 '15 at 16:12