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

[Steffen Uhlig]

Dear David, Petr, and Alain,

thank you very much for your fast responses. It's a typical 
"handbook-not-read-error" at my side. I will dig deeper into the 
plot-functions and the assignment of data. I was not aware of that the 
vector "a" is handled as a vector of factors with 10 levels. Thanks for 
your suggestions and hints!

Best regards,
/steffen


Am 26.07.2010 14:30, schrieb David Winsemius:
>
> On Jul 26, 2010, at 7:38 AM, Steffen Uhlig wrote:
>
>> 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
>
> You've gotten several answers, but none have addressed an aspect of R
> behavior that took me longer to appreciate than it perhaps should have.
> The "b" column inside the "e" data.frame is now a factor column. I
> mention that because you later referred to it as a "string" which it is
> not. It is an integer with an associated indexed level character vector.
> Many of the functions that you might think would "work" on "strings"
> will give either errors or unexpected results when applied to factors.
>
>
>>
>> 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