R help - May 2009 - Histogram frequencies with a normal pdf curve overlay

Dear List,

When I plot a histogram with 'freq=FALSE' and overlay the histogram
with a normal pdf curve, everything looks as expected, as follows:

x <- rnorm(1000)
hist(x, freq=FALSE)
curve(dnorm(x), add=TRUE, col="blue")

What do I need to do if I want to show the frequencies (freq=TRUE)
with the same normal pdf overlay, so that the plot would still look
the same?

Regards,

Jacques

platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          8.0
year           2008
month          10
day            20
svn rev        46754
language       R
version.string R version 2.8.0 (2008-10-20)

On 09-May-09 16:10:42, Jacques Wagnor wrote:
> Dear List,
> When I plot a histogram with 'freq=FALSE' and overlay the
> histogram with a normal pdf curve, everything looks as expected,
> as follows:
> 
> x <- rnorm(1000)
> hist(x, freq=FALSE)
> curve(dnorm(x), add=TRUE, col="blue")
> 
> What do I need to do if I want to show the frequencies (freq=TRUE)
> with the same normal pdf overlay, so that the plot would still look
> the same?
> 
> Regards,
> Jacques
Think first about how you would convert the histogram densities
(heights of the bars on the "density" scale) into histogram
frequencies.

  Density * (bin width) * N = frequency

where N = total number in sample. Then all you need to is multiply
the Normal density by the same factor. To find out the bin width,
take the difference between succesive values of the "breaks" component
of the histogram. One way to do all this is

  N <- 1000
  x <- rnorm(N)
  H <- hist(x, freq=TRUE)  ## This will plot the histogram as well
  dx <- min(diff(H$breaks))
  curve(N*dx*dnorm(x), add=TRUE, col="blue")

Ted.

--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 09-May-09                                       Time: 17:31:03
------------------------------ XFMail ------------------------------

Assuming a constant bin width, you need to multiply the density by
n*binwidth, where the bin width is (obviously!) the width of the
histogram bins.


>>> Jacques Wagnor <jacques.wagnor at gmail.com> 05/09/09 5:10 PM
>>>
Dear List,

When I plot a histogram with 'freq=FALSE' and overlay the histogram
with a normal pdf curve, everything looks as expected, as follows:

x <- rnorm(1000)
hist(x, freq=FALSE)
curve(dnorm(x), add=TRUE, col="blue")

What do I need to do if I want to show the frequencies (freq=TRUE)
with the same normal pdf overlay, so that the plot would still look
the same?

Regards,

Jacques

platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          8.0
year           2008
month          10
day            20
svn rev        46754
language       R
version.string R version 2.8.0 (2008-10-20)

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