[R] Plotting ordered nominal data

[Sandy Small]

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

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