Box Plot

http://www.itl.nist.gov/div898/handbook/eda/section3/boxplot.htm

The box plot identifies the middle 50% of the data, the median, and the extreme points. Box plots are formed by

Vertical axis: Response variable
Horizontal axis: The factor of interest

More specifically, we

1. Calculate the median and the quartiles (the lower quartile is the 25th percentile and the upper quartile 
   is the75th percentile).


2. Plot a symbol at the median (or draw a line) and draw a box (hence the name--box plot) between the
   lower and upper quartiles; this box represents the middle 50% of the data--the "body" of the data.
3. Draw a line from the lower quartile to the minimum point and another line from the upper quartile to  
   the maximum point. Typically a symbol is drawn at these minimum and maximum points, although 
   this is optional.