Visualizing Environmental Science

468 Graphing Appendix

that because it is not clear without further explanation,
the y-axis title on this graph includes the measurement
unit (billions) portrayed along this axis.

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LEARNING OBJECTIVES

1. Explain the difference between continuous and
   discrete data.


2. Distinguish among the following types of graphs:
   line graph, bar graph, and pie chart.

S

cientists use different types of graphs to visual-

ize and interpret different kinds of data.

Continuous data can take on an infinite num-

ber of values within a given range, and are

usually associated with measurable variables such as tem-

perature or length. For example, there are an infinite

number of measureable temperatures between 45 and 46

degrees Centigrade and lengths between 45 and 46 meters

(45.12, 45.593, etc.). In contrast, discrete data may only as-

sume a finite number of distinct values, and are usually

associated with variables that are counted, such as the num-

ber of people within a given area or the number of trees in

the forest (you can’t count 45.12 people or 45.593 trees).

One of the most common graph types is a line graph

in which one or more lines connect a series of data points

together. Line graphs are used when the y-axis portrays

continuous numeric data and the x-axis portrays either

continuous numeric data or discrete categorical data

that form a sequential series such as months or years. For

example, the line graph below (Figure A.5) shows how

the number of Earths needed to absorb humanity’s eco-

logical footprint has increased over the past five decades.

Note that we can determine the time our collective foot-

print overshot Earth’s capacity by finding the point on

the x-axis that corresponds to the point at which this line

exceeds the 1.0 value on the y-axis (around 1988).

Year

1800 1900 2000

High 10.6

Medium 9.3

Low 8.1

2050

Human population (billions)

1

2

3

4

5

6

7

8

9

10

11

Measurement

unit

Future

projections

UN Population Division 2010.

Figure A.4

1. What are data? What is a variable?


2. Explain the difference between a dependent and an
   independent variable.


3. Draw your own graph and include and label
   each of the following components: x-axis, y-axis,
   title, caption, legend, future projection, and the
   measurement units of the x- and y-axes.

1.0

Year

Number of planet Earths

0

0.5

1.5

2.0

1970 1980 1990 2000 2010

Global ecological

overshoot

Based on data from World Wildlife Fund,Living Planet Report, 2008.

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