[R] Changing x-axis on boxplot

[Peter Ehlers]

Tim,

Boxplots are nice, but I find that they can be somewhat
misleading when applied to small groups, especially in the
suggestion of spread differences. (Your real data may well
be more extensive than the ToothGrowth data and so the
following may be moot.)

I prefer stripcharts for small groups. Compare the
boxplot figure to the following (using Dennis's code
as basis):

  stripchart(len ~ dose, data = subset(ToothGrowth, supp == 'VC'),
          method = "jitter", jitter = .03,
          vertical = TRUE, at = 1:3 - 0.1,
          pch = 21, bg = "yellow", cex = 1.5,
          main = "Guinea Pigs' Tooth Growth",
          axes = FALSE,
          xlab = "Vitamin C dose mg",
          ylab = "Tooth length",
          xlim = c(0.5, 3.5), ylim = c(0, 35), yaxs = "i")
  stripchart(len ~ dose, data = subset(ToothGrowth, supp == 'OJ'),
          add = TRUE,
          method = "jitter", jitter = .03,
          vertical = TRUE, at = 1:3 + 0.1,
          axes = FALSE,
          pch = 21, bg = "orange", cex = 1.5)
  axis(1, at = 1:3, labels = c(0.5, 1, 2))
  axis(2)
  box()
  legend(2, 9, c("Ascorbic acid", "Orange juice"),
          pch = 21, pt.bg = c("yellow", "orange"), pt.cex = 1.5)


   -Peter Ehlers


On 2010-09-26 4:48, Dennis Murphy wrote:
> Hi:
> Here are a couple of ways:
>
> (1) Base graphics: add argument axes = FALSE to both plots, then add axes
>
> boxplot(len ~ dose, data = subset(ToothGrowth, supp == 'VC'),
>          boxwex = 0.25, at = 1:3 - 0.2,
>          col = "yellow",
>          main = "Guinea Pigs' Tooth Growth",
>          axes = FALSE,
>          xlab = "Vitamin C dose mg",
>          ylab = "Tooth length",
>          xlim = c(0.5, 3.5), ylim = c(0, 35), yaxs = "i")
> boxplot(len ~ dose, data = subset(ToothGrowth, supp == 'OJ'),
>          add = TRUE,
>          boxwex = 0.25, at = 1:3 + 0.2,
>          axes = FALSE,
>          col = "orange")
> axis(1, at = 1:3, labels = c(0.5, 1, 2))
> axis(2)
> legend(2, 9, c("Ascorbic acid", "Orange juice"),
>         fill = c("yellow", "orange"))
> box()
>
>
> (2) ggplot2  (just for fun :)
>
> library(ggplot2)
> g<- ggplot(ToothGrowth, aes(x = factor(dose), y = len))
> g + geom_boxplot(aes(fill = supp), position = position_dodge(width = 0.9)) +
>      scale_fill_manual('', breaks = c('OJ', 'VC'),
>                            values = c('orange', 'yellow'),
>                            labels = c('Orange juice', 'Ascorbic acid')) +
>      theme_bw() +
>      opts(legend.position = c(0.8, 0.2), legend.text = theme_text(size = 12))
> +
>      opts(panel.grid.major = theme_blank()) +
>      xlab('Vitamin C dose (mg)') + ylab('Tooth length') +
>      opts(title = "Guinea pigs' tooth growth") +
>      opts(plot.title = theme_text(size = 16))
>
> HTH,
> Dennis
>
>
> On Sat, Sep 25, 2010 at 11:13 PM, Tim Clark<mudiver1200 at yahoo.com>  wrote:
>
>> Dear List,
>>
>> I am creating a boxplot with two subsets, very similar to the example by
>> Roger
>> Bivand at ?boxplot (reproduced below).  I am trying to change the labels on
>> the
>> x-axis to have one number to cover both subsets.  I can do this in other
>> plots
>> by using axis=FALSE followed by a separate axis() command.  I have also
>> tried
>> variations in the names= argument but can't get it to work.  Ideally I
>> would
>> like tickmarks on either side of each factor with the number for the level
>> centered between the two tick marks.  Any suggestions?
>>
>> Thanks,
>>   Tim
>>
>> Example:
>>
>> boxplot(len ~ dose, data = ToothGrowth,
>>          boxwex = 0.25, at = 1:3 - 0.2,
>>          subset = supp == "VC", col = "yellow",
>>          main = "Guinea Pigs' Tooth Growth",
>>          xlab = "Vitamin C dose mg",
>>          ylab = "tooth length",
>>          xlim = c(0.5, 3.5), ylim = c(0, 35), yaxs = "i")
>> boxplot(len ~ dose, data = ToothGrowth, add = TRUE,
>>          boxwex = 0.25, at = 1:3 + 0.2,
>>          subset = supp == "OJ", col = "orange")
>> legend(2, 9, c("Ascorbic acid", "Orange juice"),
>>         fill = c("yellow", "orange"))
>>
>>
>>   Tim Clark
>>
>> Marine Ecologist
>> National Park of American Samoa
>>
>>
>>