R Graphics Cookbook

Chapter 15. Getting Your Data into Shape

When it comes to making graphs, half the battle occurs before you call any graphing commands. Before you pass your data to the graphing functions, it must first be read in and given the correct structure. The data sets provided with R are ready to use, but when dealing with real-world data, this usually isn’t the case: you’ll have to clean up and restructure the data before you can visualize it.

Data sets in R are most often stored in data frames. They’re typically used as two-dimensional data structures, with each row representing one case and each column representing one variable. Data frames are essentially lists of vectors and factors, all of the same length, where each vector or factor represents one column.

Here’s the heightweight data set:

library(gcookbook) # For the data set
heightweight

sex ageYear ageMonth heightIn weightLb
  f   11.92      143     56.3     85.0
  f   12.92      155     62.3    105.0
 ...
  m   13.92      167     62.0    107.5
  m   12.58      151     59.3     87.0

It consists of five columns, with each row representing one case: a set of information about a single person. We can get a clearer idea of how it’s structured by using the str() function:

str(heightweight)

'data.frame':   236 obs. of  5 variables:
 $ sex     : Factor w/ 2 levels "f","m": 1 1 1 1 1 1 1 1 1 1 ...
 $ ageYear : num  11.9 12.9 12.8 13.4 15.9 ...
 $ ageMonth: int  143 155 153 161 191 171 185 142 160 140 ...
 $ heightIn: num  56.3 62.3 63.3 59 62.5 62.5 59 56.5 62 53.8 ...
 $ weightLb: num  85 105 108 92 112 ...

The first column, ...