[R] pch and curve plotting
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Wed Aug 9 18:09:07 CEST 2000
On Wed, 9 Aug 2000, Margaret Riordan wrote:
> Hi all,
> I'm having two small problems and was wondering if you could help me
> Problem 1:
> When I use pch to place a symbol onto a plot it seems to place the point
> a little off the exact location. I have read the help file for this
> which says to use "." as the plotting character, which will then produce
> centered points but I am unclear as to how to do this.
pch="." But we have tried hard to get the symbols in the right
places, so which R version and which graphics device is this?
plot(1:10, 1:10, type="n")
for(i in 1:10) for(j in 1:10) points(i, j, pch = ".")
is correct (to 0.5 pixel) on my system) and so is
for(i in 1:10) for(j in 1:10) points(i, j, pch = 16)
(yes, I do know how to vectorize this!)
> Problem 2: How do I get R to draw a curve through a number of data
> points rather than a straight line. From the description of the
> functions scatter.smooth and loess.smooth, I think they are what I'm
> looking for,
> but when I try to run them in R Version 1.1.0, it returns the following
> Error in sdgraph() : couldn't find function "scatter.smooth"
They are in library modreg. You should not have got help on them in
1.1.0 without doing library(modreg) first.
However, they fit a smooth curve through a scatter plot, not through
the data points themselves (smoothing, not interpolation).
There are many other possibilities, and I would tend to prefer
smooth.spline (also in modreg) as it automatically chooses the
degree of smoothing. Then there are regression splines, running
lines smoothers (supsmu), kernel smoothers, .... V&R3 and
its on-line statistical complements have a catalogue and comparisons.
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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