[R] intervals from cut() as numerics?

[Gavin Simpson]

Hi,

Given some example data:

dat <- seq(4, 7, by = 0.05)
x <- sample(dat, 30)
y <- sample(dat, 30)
error <- x - y

I have broken the rage of x into 10 groups and I can calculate the bias
(mean(error)) for each of these 10 groups:

groups <- cut(x, breaks = 10)
max.bias <- aggregate(error, list(group = groups), mean)
max.bias
       group          x
1    (4,4.3] -0.7750000
2  (4.3,4.6] -0.9750000
3  (4.6,4.9] -0.6750000
4  (4.9,5.2] -0.9125000
5  (5.2,5.5]  1.0500000
6  (5.5,5.8]  0.3166667
7  (5.8,6.1]  0.1375000
8  (6.1,6.4]  1.1833333
9  (6.4,6.7]  0.2500000
10   (6.7,7]  1.7000000

Which is fine. Now I am producing a plot of the residuals vs observed
and I want to draw line segments from e.g. (4,4.3] at height =
-0.7750000, but for all 10 groups, like so:

plot(x, error, type = "n")
abline(h = 0, col = "grey")
panel.smooth(x, error)
arrows(4, -0.775, 4.3, -0.775, length = 0.05, angle = 90, code = 3)

The problem is getting the range/interval for each group from (4,4.3],
so I can automate this.

Anyone think of a way to do this - happy to change the way the groups
are generated if cut() is not the right tool for the job here.

Many thanks,

Gav
-- 
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Gavin Simpson                     [t] +44 (0)20 7679 0522
ECRC                              [f] +44 (0)20 7679 0565
UCL Department of Geography
Pearson Building                  [e] gavin.simpsonATNOSPAMucl.ac.uk
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