Graphing

In science, it is often extremely useful to plot a graph in order to visualize your data and obtain information. Here are some key points on graphing – be careful as this is where most people lose points! Be sure to read the directions on how to draw your graphs for any given experiment.

• Make sure that you know in advance what needs to be plotted on each graph, and plan accordingly. The independent variable always goes on the x-axis (horizontal axis) and the dependent variable always goes on the y-axis (vertical axis).
• Every graph should be properly labeled. Proper labeling includes a graph title, axes titles (with appropriate units), and axes numbering for each major division.

If there is more than one data series plotted on your graph, you should use different symbols or ink color for each series, and include a legend to indicate which symbols or ink color represent which data set.

• Spread out the points across the entire page. I expect you to use one sheet of graph paper per graph in general; postage-stamp sized graphs are unacceptable. No exceptions should be made (at this level) for computer-generated graphs. The key point is that anyone should be able to visualize the trends and check the fitting.

• Best fit lines should represent either a straight line or a smooth curve, as appropriate for the data. Never join dots with lines! If you are trying to plot a straight line, it is critical that you use a sharp pencil and a 12″ ruler. Untidy best-fit lines will be penalized.

Smooth curves should always be drawn in a way that represents the data and chemistry appropriately. It should be both physical (reflect a plausible physical relationship) and be a reasonable representation of the data. Please consult your instructor if you are having trouble with this.

• Markers should be clear and not too large. A cross or dot will suffice. Make sure that you know what you need to plot on each graph – there may be multiple axes to plot on the same graph.
• If there are multiple series on your graphs then please make sure that these can be distinguished clearly and that there is a legend or series labels identifying each of these series.
• Make sure all points required are on the graph.

When taking the slope of a best fit straight line, please take two points straight off the best fit line (not two data points) or use the Excel display equation option – with the equation printed on the graph (which suffices as showing the work for calculating the slope provided that you reference it in your sample calculations and state which number you used for the slope). When you use two data points to determine the slope of the graph, you’re essentially determining the slope based on two data points rather than all of the data points.