Dot Plots and Stem Plots

Dot Plots

To demonstrate the construction of dot plots (or dot charts) we will use the data in the example below.

Example: The table below provides data on the approximate number of drug related arrests (in thousands) from 2001 to 2010.

Year	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010
Number of Arrests	35	30	29	31	30	31	30	29	32	31

Source: www.justice.gov

First we'll need to create a data set.

> arrests = c(35, 30, 29, 31, 30, 31, 30, 29, 32, 31)

To create a dot plot we will use the command dotchart.

> dotchart(arrests)

If we want to add more information to our plot we can include the main and xlab arguments.

> dotchart(arrests, main = "Drug Related Arrests - 2001 to 2010", xlab = "Number of Arrests (in thousands)")

Stem Plots

To demonstrate the construction of stem plots (or stem-and-leaf plots) will use the data in the example below.

Example: The table below provides data on the approximate number of drug related arrests (in thousands) from 2001 to 2010.

Celestial Body	Number of Successful Probes
Sun	17
Mercury	3
Venus	27
Mars	24
Jupiter	10
Saturn	3
Uranus	1
Neptune	1
Asteroids	11
Comets	13
Moon	52

Source: wikipedia.org

First we'll need to create a data set.

> probes = c(17, 3, 27, 24, 10, 3, 1, 1, 11, 13, 52)

To create a stem plot we will use the command stem.

> stem(probes)

  The decimal point is 1 digit(s) to the right of the |

  0 | 11330137
  2 | 47
  4 | 2

Notice, though, that the "stems" jump by two.  To force the "stems" to step by one we will include the argument scale = 2.

> stem(probes, scale = 2)

  The decimal point is 1 digit(s) to the right of the |

  0 | 1133
  1 | 0137
  2 | 47
  3 | 
  4 | 
  5 | 2

The value for scale will at times require experimentation, with the end goal of having the "stems" step by one.