[R] Plotting ordered nominal data

[Marc Schwartz]

on 08/01/2008 10:21 AM Sandy Small wrote:
> Hi
> I'm sure this question has been asked before but I can't find it in the 
> archives.
> I have a data frame which includes interval and ordered nominal results. 
> It looks something like
> 
> "Measured"      "Eyeball"
> 46.5   Normal
> 43.5   Mild
> 56.2   Normal
> 41.1   Mild
> 37.8   Moderate
> 12.6   Severe
> 17.3   Moderate
> 39.1   Normal
> 26.7   Mild
> NULL   Normal
> 27.9   NULL
> 68.1   Normal
> 
> I want to plot the Measured value against the "Eyeball" value but if I 
> simply plot it the "Eyeball" values are plotted in alphabetical order. I 
> do not want to change the "names" as "Normal, Mild, Moderate, Severe" 
> are standard but I want to plot them in the order "Normal", "Mild", 
> "Moderate", "Severe" so that the trend (or not) is obvious.
> 
> Any help would be much appreciated.
> Many thanks
> Sandy

We are going to need a bit more info.

What type of plot?

   Point estimates of the means per level of severity?

   Boxplot?


Some possibilities:

# Read in the data from the clipboard, converting the "NULL"s to NAs
DF <- read.table("clipboard", header = TRUE, na.strings = "NULL")

 > DF
    Measured  Eyeball
1      46.5   Normal
2      43.5     Mild
3      56.2   Normal
4      41.1     Mild
5      37.8 Moderate
6      12.6   Severe
7      17.3 Moderate
8      39.1   Normal
9      26.7     Mild
10       NA   Normal
11     27.9     <NA>
12     68.1   Normal


# Change the factor ordering and include NA as a level
DF$Eyeball <- factor(DF$Eyeball,
                      levels = c("Normal", "Mild", "Moderate", "Severe",
                                 NA),
                      exclude = NULL)

# Do a boxplot
boxplot(Measured ~ Eyeball, data = DF)


# Plot means by severity
Res <- tapply(DF$Measured, list(DF$Eyeball), mean, na.rm = TRUE)

plot(Res, pch = 19, ylab = "Mean", xlab = "Severity", xaxt = "n")

axis(1, at = 1:5, paste(names(Res)))


See ?factor, ?plot.default, ?boxplot and ?axis

HTH,

Marc Schwartz