Transformations axis vs values

log transformation: axis vs values

• sometimes it is useful to transform the values
• in ggplot you have the choice between transforming the values or the axis
• here is an example to compare both
• the data set movie is part of the ggplot package, additional used are the gridExtra package (grid.arrange), the scales package (labels formatter "dollar")
• in the beginning a cut the year var in 15 even spaced intervals
• on the lefthandside is the plot with the transformation of the axis, on the righthandside with transformation of values

movies$years <- cut(movies$year,breaks=15)

p <- ggplot(movies, aes(x=years,y=budget,colour=years)) +
## bar filling
  geom_boxplot(fill="black") + 
## add mean points
  stat_summary(fun.y="mean",geom="point") + 
 ## customize tick labels
  opts(axis.text.x = theme_text(angle=270,hjust=0,vjust=1)) +
## get rid of the legend
  opts(legend.position="none") 

## log transformation of the y axis
p1 <- p + scale_y_continuous(labels=dollar,trans="log")

## compare with
## here we do the transformation while mapping:
p2 <- ggplot(movies, aes(x=years,y=log(budget),colour=years)) +
  geom_boxplot(fill="black") +
  stat_summary(fun.y="mean",geom="point") +
  opts(axis.text.x = theme_text(angle=270,hjust=0,vjust=1)) +
  opts(legend.position="none")

## plot both graphs on one page
grid.arrange(p1,p2,ncol=2)

Boxplot with Means

boxplots as alternative to barplot with error bars

• a more informative alternative to this barplot with errorbars
• using the movies example data set (part of the ggplot2 package)
• cut the year variable in 15 equal intervals gives the new variable years
• map this new year var to x, budget to y and colour also to years (aes(x=years,y=budget,colour=years))
• add a layer boxplot, set the fill colour for the bars to black
• format the y axis (use dollar formatting)
• change the x axis tick labels with a text theme: rotate the tick labels (angle=270), position adjustment with hjust and vjust
• we do not need a legend for he colours because the information is contained in the x axis, so get rid of it: opts(legend.position="none")

movies$years <- cut(movies$year,breaks=15)
ggplot(movies, aes(x=years,y=budget,colour=years)) +
  geom_boxplot(fill="black") +
  scale_y_continuous(labels=dollar) +
  stat_summary(fun.y="mean",geom="point") +
  opts(axis.text.x = theme_text(angle=270,hjust=0,vjust=1)) +
  opts(legend.position="none")

Warnmeldungen:
1: Removed 53573 rows containing non-finite values (stat_boxplot). 
2: Removed 53573 rows containing missing values (stat_summary).