[R] Overlying a Normal Dist in a Barplot

From: Bret Collier <bret_at_tamu.edu>
Date: Sat 09 Jul 2005 - 05:22:14 EST


R-Users,
Hopefully someone can shed some light on these questions as I had little luck searching the archives (although I probably missed something in my search due to the search phrase). I estimated multinomial probabilities for some count data (number successful offspring) ranging from 0 to 8 (9 possible response categories). I constructed a barplot (using barplot2) and I want to "overlay" a normal distribution on the figure (using rnorm (1000, mean, sd)). My intent is to show that using a mean(and associated sd) estimated from discrete count data may not be a valid representation of the distribution of successful offspring.

Obviously the x and y axes (as structured in barplot2) will not be equivalent for these 2 sets of information and this shows up in my example below.

  1. Is it possible to somehow reconcile the underlying x-axis to the same scale as would be needed to overly the normal distribution (e.g. where 2.5 would fall on the normal density, I could relate it to 2.5 on the barplot)? Then, using axis (side=4) I assume I could insert a y-axis for the normal distribution.
  2. Is lines(density(x)) the appropriate way to insert a normal distribution into this type of figure? Should I use 'curve'?

If someone could point me in the right direction, I would appreciate it.

TIA, Bret

Example:

testdata

0    0.196454948
1    0.063515510
2    0.149187592
3    0.237813885
4    0.282127031
5    0.066469719
6    0.001477105
7    0.001477105
8    0.001477105


x<-rnorm(1000, 2.84, 1.57)
barplot2(testdata, xlab="Fledgling Number",

             ylab="Probability", ylim=c(0, 1), col="black", 
             border="black", axis.lty=1)
lines(density(x))

--Version--
platform i386-pc-mingw32

arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    2              
minor    0.1            
year     2004           
month    11             
day      15             

language R

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