How to Draw Graphs

Data and Data Analysis

In order to analyse the data obtained during experiments it is often necessary to convert that data into different representations. One type of representation is a graph. A few examples are given here.

How to Draw Graphs in Science

For all graphs plotted from experimental data it is important to remember that you should never connect the dots. Data will never follow a line or curve perfectly. By obtaining multiple experimental data points any discrepancies in each data point can be removed. The line added after the points are plotted should be a best fit line that can then be used to more accurately determine further information.

Features of graphs you plot:

• An appropriate scale is used for each axis so that the plotted points use most of the axis/space (work out the range of the data and the highest and lowest points).

• The scale must remain the same along the entire axis and should use easy intervals such as 10's, 20's, 50's. Use graph paper for accuracy.

• Each axis must be labelled with what is shown on the axis and must include the appropriate units in brackets, e.g. Temperature (\(^{\circ}\)C), time (seconds), height (cm).

• The independent variable is generally plotted along the x-axis, while the dependent variable is generally plotted along the y-axis.

• Each point has an x and y co-ordinate and is plotted with a symbol which is big enough to see, e.g. a cross or circle.

• A best fit line is then added to the graph.

• Do not start the graph at the origin unless there is a data point for (0,0), or if the best fit line runs through the origin.

• The graph must have a clear, descriptive title which outlines the relationship between the dependent and independent variable.

• If there is more than one set of data drawn on a graph, a different symbol (and/or colour) must be used for each set and a key or legend must define the symbols.

• Use line graphs when the relationship between the dependent and independent variables is continuous.

• For a line graph, you can draw a line of best fit with a ruler. The number of points are distributed fairly evenly on each side of the line (see figure below).

A straight line graph of the change in temperature with time.

• With an exponential graph (when the points appear to be following a curve) you can draw a best fit line freehand (see figure below).

A graph with an exponential best fit line.

Remember that without units much of our work as scientists would be meaningless. We need to express our thoughts clearly and units give meaning to the numbers we measure and calculate. Depending on which units we use, the numbers are different. For example if you have 12 water, it means nothing. You could have 12 ml of water, 12 litres of water, or even 12 bottles of water. Units are an essential part of the language we use. Units must be specified when expressing physical quantities.