Drawing Scatter Plots & Line Graphs

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What is a Scatter Plot and Line Graph?

Scatter plots and line graphs are the most commonly used graphs in Science. They are used when two variables are being considered, and one variable is dependent on the other. The graph shows the relationship between the two variables.

Important note: Always use a pencil, ruler and grid paper for your graphs!

Variables and Points

The independent variable, which is set by the experimenter, is always shown on the x-axis. The dependent variable, which is the variable measured in the experiment, is always shown on the y-axis.

The data are plotted on the graph as a series of points. Each point should be drawn in pencil as a small circle or 'x'. The points may be joined with a ruled line. There are two ways to join the points:

Line Graph

Rule a line from each point to the next. This creates a line graph (Grapn A) and may give a jagged line. The graph is intended to highlight the known points and to include all the points. It shows the overall trend, and is not meant to predict the value of the points between the plotted data.

Line of Best Fit

Join the points with a single, smooth, straight or curved line. This creates a trend line or line-of-best-fit graph from the scatter plot (Graph B). The trend line does not have to pass through every point. A trend line is used when there is an obvious trend between the variables, such as a mathematical relationship.

When you have missing data, leave a gap for it (graph C). Ensure that the axes are complete (do not skip values), and do not join the dots for data t have missing data points between them.

Sometimes, when you collect data, there may be one point that is way out. It is clearly wrong. This is called an outlier (graph D). An outlier is often caused by a mistake in measuring or recording, or from a random error in the measuring equipment. If you have an outlier, you should include it in your graph, but ignore it when joining the dots to obtain a graph line.

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