[R] boxplots with multiple numerical variables

[Marc Schwartz]

>-----Original Message-----
>From: r-help-bounces at stat.math.ethz.ch 
>[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Rishabh Gupta
>Sent: Friday, March 14, 2003 4:10 PM
>To: r-help at stat.math.ethz.ch
>Subject: [R] boxplots with multiple numerical variables
>
>
>Hi all,
>   I have a question regarding the boxplot function. The data 
>I am working on has 1 grouping variable (G) and it has many 
>numerical variables (V1, V2, V3, V4, Vx, etc). What I would 
>like to do is create a boxplot where the Y-axis represents the 
>numerical values of variable V1...Vx (all the variables have 
>the same range). The X-axis needs to represent the G-V 
>combination. So suppose the possible values for G are a, b and 
>c, Then along the x-axis there would be a boxplot for each of 
>the combinations:
>
>  V1Ga, V1Gb, V1Gc, V2Ga, V2Gb, V2Gc, V3Ga, V3Gb, 
>V3Gc,.....VxGa, VxGb, VxGc, etc ie
>  all values of V1 where the G values are a, all values of V1 
>where the G values are b, etc In addition, if possible, it 
>would be nice if each G value would have a a different colour 
>on the plot so that they could be seen more clearly.
>
>I'm not sure whether such a function already exists within R 
>or whether it would have to be written. Either way, I would 
>appreciate it very much if somebody could help and give me 
>some advice as to how I can achieve this.
>
>Many Thanks
>
>Rishabh

If I understand correctly, you basically want to generate a grouped
boxplot, where each of the numeric variables are in groups of three
(a, b and c) across the x-axis.

The final example in ?boxplot does this, though for groups of 2 and it
uses a formula method. You would just need to modify the code in the
example adding a third call to boxplot() with the 'add = TRUE'
argument set as in the second call.

Note the use of the 'at' argument, which enables you to pick
mid-points for the groups and then draw the boxplots at offsets from
the mid-points.  In the case of groups of three, you would use the
mid-points and then the mid-points minus and plus an offset.

You then also use the 'boxwex' argument, to adjust the width of the
boxplots to fit the groupings.

Finally, for the colors, you can use a different 'col' argument in
each call to boxplot, which would yield three colors, one for each of
the three boxplots in each grouping.

HTH,

Marc Schwartz