Simple histogram plot wrong?

Question

test <- rep(5,20)
hist(test,freq=FALSE,breaks=5)

The vector contains 20 times the value 5. When I plot this with freq=FALSE and breaks=5 I expect to see 1 bar at x=5 with height = 1.0, because the value 5 makes up 100% of the data.

Why do I instead see 1 bar that ranges from x=0 to x=5 and has height = 0.2 ??

score 4 · Accepted Answer · edited May 23 '17 at 10:29

4

hist plots an estimate of the probability density when freq=FALSE or prob=TRUE, so the total area of the bars in the histogram sums to 1. Since the horizontal range of the single bar that is plotted is (0,5), it follows that the height must be 0.2 (5*0.2=1)

If you really want the histogram you were expecting (i.e. heights correspond to fraction of counts, areas don't necessarily sum to 1), you can do this:

 h <- hist(test,plot=FALSE)
 h$counts <- h$counts/length(test)
 plot(h)

Another possibility is to force the bar widths to be equal to 1.0, e.g.

 hist(test,freq=FALSE,breaks=0:10)

Or maybe you want

 plot(table(test)/length(test))

or

 plot(table(test)/length(test),lwd=10,lend="butt")

? See also: How do you use hist to plot relative frequencies in R?

edited May 23 '17 at 10:29

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answered Jan 12 '14 at 15:18

Ben Bolker

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so how do i get 1 bar at the value 5 of height 1.0 ? – user3182532 Jan 12 '14 at 15:21
no I didn't. but now I have tried that and I still get the same result ... ? – user3182532 Jan 12 '14 at 15:26
really? are you sure the height isn't now 1.0? – Ben Bolker Jan 12 '14 at 15:30
yes the height is 1.0, but the width of the bar is too large. i have 1 big square which ranges from 0 to 5 on the x-axis. i need 1 bar at the value 5, which indicates that the value 5 makes up 100% of the data. – user3182532 Jan 12 '14 at 15:32
you might prefer my second suggestion above. – Ben Bolker Jan 12 '14 at 15:33
1

@user3182532, if BenBolker's answer addressed correctly your question, you might consider accepting his answer by clicking on the green checkmark under the vote arrows. Tks. – Andre Silva Jan 12 '14 at 19:52

Simple histogram plot wrong?

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