[R] Is there a problem with lattice axes?

[(Ted Harding)]

After answering a previous post by James Rome:
"Re: [R] Why do histogram bars vary their width?"

I noticed that the lateral axis scales in the lattice histogram
which I used for illustration are inconsistent with the frequencies.
The example was:

set.seed(54321)
 X <- c(rnorm(100),rnorm(50,1.0,0.5),rnorm(25,-1.0,0.5),rnorm(10))
  F <- factor(c(rep(0,100),rep(1,50),rep(2,25),rep(3,10)))
# The default:
  histogram(~X | F)
# Your choice (number of breaks)
  histogram(~X | F, breaks=10)
# Explicit specification of break-points:
  histogram(~X | F, breaks=0.5*(-6:6))

In the third of these, you will see that the frequency scales are
marked at marked at 0,10,20,30,40,50 throughout. This is appropriate
for the bottom-left histogram (Level of F=0, N=100), and the
displayed frequencies do indeed (by eye) seem to add up to 100.
It is certainly not right for the top-right histogram (F=3,N=10),
where the displayed scale needs to be 0,1,2,3 (giving 1+3+5+1=10),
i.e. divided by 10; but at least this one is readily re-scalable
"by eye". However, it is neither right nor readily re-scalable
for the other two:

  F=1, N=50, apparent sum = 2+10+44+22+18+4=100
  F=2, N=25, apparent sum = 8+32+48+12     =100

Clearly, while in principle it would be possible to have
  A: an axis lower-left  (F=0,N=100) scaled 0,10,  20,  30,  40,  50
  B: an axis lower-right (F=1,N= 50) scaled 0, 5,  10,  15,  20,  25
  C: an axis upper-left  (F=2,N= 25) scaled 0, 2.5, 5.0, 7.5,10.0,12.5
  D: an axis upper-right (F=3,N= 10) scaled 0, 1,   2,   3,   4,   5
in this 2x2 case, it could not work for say a 3x3 lattice.

The right thing to do when there is a common vertical scale
along a row is to scale each plot itself accordinng to the
vertical scale for the row. Instead, it seems to have chosen
a vertical scale for each histogram as plotted so that the
histiogram bars will nicely fill the panel window in each case,
disregarding the vertical scaling which has been determined
according to the histogram with the largest N.

Thus one could have a scale 0,10,20,30,40 for the bottom row,
with the lower right-hand histogram bars being half the height
they have as displayed by the above, and either
  A: the same scale for the upper row as for the bottom row,
     but with the left-hand histogram bars 1/4 the height
     and the right-hand histogram bars 1/10 the height
or
  B: a scale 0,2.5,5.0,7.5,10 with the left-hand histogram
     bars just as they are now, and the right-hand histogram
     bars 1/2.5 the hright.

Or maybe there is an option lurking somewhere in the lattice
documentation which looks after this sort of thing -- but I
have not been able to find it despite have spent some time
searching.

R version 2.10.0 (2009-10-26)
lattice version: 0.17-26 (2009/10/05)

Ted.



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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 26-Dec-09                                       Time: 20:07:51
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