Tutorials

Rules for Graphing

A graph presents numerical information in a pictorial form. Three kinds of graphs are commonly used.

• line graph

• used to display the relationship between continuous data

• demonstrates a progression of values or shows how one variable changes in relation to another variable; for example, growth of a child over time

Note: When mathematical equations are graphed, a line is drawn.

• bar graph

• used to display discrete or discontinuous data

• consists of parallel bars whose lengths are proportional to quantities given in a set data

Note: The items compared are plotted along the horizontal axis, and appropriate measurement is plotted along the vertical axis. The numbers and types of protists in a lake may be illustrated in a bar graph.

• scatter graph

• used to display the relation between two random variables

• length of an experiment and number of bacteria grown may be illustrated in a scatter graph

Note: A line of best fit may be shown on the graph.

Generally, the same rules for graphing apply to all three types of graphs.

• The graph must have a title. The title represents the relationship between the two variables.

• The manipulated (independent) variable is diagrammed on the horizontal x -axis.

• The responding (dependent) variable is diagrammed on the vertical y -axis.

• Each axis is specifically labelled according to the variable it represents, and units are provided with equal increments. The scale does not have to be the same on both axes, but the scales must accommodate the ranges of the two variables.

• Data is plotted. The graph may show exact numbers or a general relationship. A best fit line is often used in line and scatter graphs where a straight line is a rough linear estimate with values equally divided above and below the line.

• A legend may be used to identify individual lines on a multi-line graph.