[R] Plot of a subset of a data.frame()

[Steffen Uhlig]

Hello,

my data.frame is sort of a collection of process values, i.e. huge 
run-chart. It consists of a time-stamp in the first column (date as 
string), factors in the following columns (used for subset-filtering), 
and some process-data columns.
Hereafter, two examples are listed, showing the problems that occour 
during print:

At first the example, that works fine:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a = c(1:10) 		# create a vector of integers
b = rep(c("a","b"),5)	# create a vector of chars, used
			# as factor-levels	
d = rnorm(10)		# some random numbers
e = data.frame(a,b,d)	# connect to a data.frame

e.1 = subset(e, b=="a")	# create two subsets
e.2 = subset(e, b=="b")
plot(d~a, e.1, pch=3, col=2) # plot first data-subset
points(d~a, e.2, pch=4, col=3) # plot the 2nd one

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
all looks fine in theses plots.


However, changing the content of vector "a" to a set of strings the 
following happens:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a = c("a","b","c","d","e","f","g","h","i","j")
e = data.frame(a,b,d)       # re-build data.frame

e.1 = subset(e, b=="a")     # create two subsets
e.2 = subset(e, b=="b")
plot(d~a, e.1, pch=3, col=2)
points(d~a, e.2, pch=4, col=3)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The plot-command produces horizontal lines instead of dots. This seems 
to happen when the x-axis contains strings rather than numbers. is there 
a way out?

Best regards,
/Steffen
-- 
Steffen Uhlig, PhD
Mechatronik und Sensortechnik
HTW des Saarlandes
Goebenstraße 40
66117 Saarbrücken

Tel.: +49 (0) 681 58 67 274