Preparing Graphs1

Author: J. M. McCormick

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Last Update: May 8, 2013

General Considerations

When we prepare a graph the independent variable is always on the “x-axis”, and the dependent variable is always on the “y-axis”. We indicate which variable is which by saying as a function of or “versus”, with the dependent variable coming first, and the independent variable coming second. So if someone says, “volume was plotted as a function of mass” or “the volume is plotted versus mass,” it means that mass was on the x-axis and volume was on the y-axis. Watch what you say/write as there is only one correct usage!

All graphs must have axis labels. The axis labels have two parts: the first is the name of the parameter, and the second is the unit. The axis labels can have one of three formats: parameter name (unit); parameter name, unit; parameter name/unit. For example, if the parameter was temperature and it was measured in Kelvin, then the axis label could be Temperature (K), or Temperature, K or Temperature/K.

Devise a scale for each axis so that the tick mark labels end in a “0” or a “5”. If this is not possible, use a scale so that the last digit in the tick mark labels is an even number. Tick mark labels with the last digit being an odd number, other than “5”, are almost never used. The axes do not need to start at zero. For example, if all the x values occur between 400 and 600 nm, a graph of these data could start at 400 nm. The key to preparing good graphs is selecting a scale that shows all of the data and minimizes large regions of blank space. The one exception is when you need to extrapolate back to a certain value, but the data are not necessarily close to that value. In this case, you would want to have the value to which you will extrapolate shown on the graph, even though there may be some blank space.

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The number of significant figures in the tick marks is usually less than that in the original data. For example, if our temperature axis has values between 200 and 320 K with each individual point measured to the nearest 0.01 K, we would not write the tick marks as 200.00, 220.00, etc., but rather 200, 220, etc. (see Fig. 1 and Fig. 2).

Graphs that will appear in a notebook must also have a title. The title should concisely tell the reader what is in the graph. Avoid the obvious use of “vs.” or “versus” or the word “plotted” in the title. Graphs that will appear as a figure in a publication or in a formal laboratory report will not have a title (the information is given in the figure caption).

Hand-Drawn Graphs

For hand-drawn graphs in the notebook choose a scale so that the graph fills most, if not all of the page. Use a small, dark dot for each data point. Draw a circle around the point to help the reader locate the actual data point. The radius of the circle usually approximates the uncertainty in the point unless this gives a circle that is too large. With hand-drawn graphs, one usually does a linear regression “by eye”, which means that a ruler is used to put a line through the data such that all points lie as close as possible to the line. The slope and intercept can then be determined from the line itself using points on the line at the extremes of the graph to maximize the precision.

Figure 1. A properly executed hand-drawn graph.

Spreadsheet Graphs

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With a spreadsheet it is much easier to prepare graphs, but it is also much easier to produce a poor quality graph. To assure that your graphs are correctly prepared (e. g., look good and are easy to understand by the reader), follow these standard procedures:

-Most of the graphs that you will prepare in a chemistry class are called “XY Scatter” plots in Excel. The other formats will be used sparingly because they are generally not useful in chemistry. -When creating the graph, make the chart occupy a new sheet; do not create it in the worksheet containing the data. -Make all lines (the axes, the tick marks, the box around the plot area and any data lines) black with medium intensity (third choice from the top under “Weight” in the format boxes). -The plot area should have no fill color (not the default setting) and no box around it. -Turn off all gridlines (not the default setting), unless told otherwise by your instructor. -Legends showing what a given symbol means are useful, but not always needed (e. g., in a formal laboratory report this information is given in the caption that accompanies the figure). Be sure that your selection of lines and legend titles clearly distinguish between multiple data sets and fits. -A title should be placed at the top of the graph if the graph is to be placed in the laboratory notebook. This helps the reader immediately know what the graph is. The title should be a concise description of what is being graphed (e. g., “Pressure as a Function of Temperature for Nitrogen”). Usually you do not need to describe in the title the units used in the graph, but there are some instances where this is necessary. Avoid using the words “versus”, “vs.” and “plotted” in the title. The title is omitted when the graph will be used as a figure in a publication or formal laboratory report. This is because the information normally put in the title will be included in the figure caption. -Watch your tick marks. The spreadsheet automatically selects the spacing, which may not be appropriate for your graph (see General Considerations). – Be careful with the use of color to distinguish different data sets on the same graph. Yellow and light blue do not show up very well when printed either on color or black and white printers. Do not distinguish different data sets by color if you do not have a color printer. In general, it is best to dispense with color entirely and make all lines and symbols black (or at least a uniform dark color). -Data are always shown as symbols and fits to the data are shown as lines or curves. Do not connect the data points with lines. This will confuse the reader as to whether these lines represent a fit, or not. -For graphs that will be placed in a notebook, you can include the equation of a best-fit line and the R2 value for the fit in a legend (but remember that this information should also be written in the notebook as part of the graph’s description, in case the graph is removed). However, for graphs that will be submitted for publication or used in a formal laboratory report, this information is not shown on the graph itself. Rather, it is placed either in the figure caption or in the body of the text itself. -Sometimes a fit is not required, or cannot be done, but you still want to show a trend in the data. In this case a smooth line that passes through the data as an “aid to the eye” is used, and is so indicated in the text accompanying the graph. -A 16-point font for axis labels and graph titles and a 14-point font for the tick mark labels are better choices for general work than the default settings. The font used for the title should match that used for the axis labels. -Depending on the final size of the graph, you might want to increase the size of the symbols to 9 point. -Most spreadsheets automatically select the number of decimal places on the tick labels from the data, but not always correctly. For example, if your data were between 50 and 100, you would want tick labels of 50, 60, 70, 80, 90, and 100. But if the data in the spreadsheet are set to two decimal places, most spreadsheets would make the labels 50.00, 60.00, 70.00, 80.00, 90.00 and 100.00. Although this is not necessarily incorrect, the convention is to show the minimum number of decimal places on the tick labels.

Remember that the default settings in a spreadsheet may not fit your needs. Therefore, it is critical that you know how to change the settings (see the Guide to Excel).

Figure 2. A graph that was properly prepared for a laboratory notebook using a spreadsheet.

References

1. Click here to obtain this file in PDF format (suitable for printing). 2. Dodd, J. S., Ed. The ACS Style Guide: a Manual for Authors and Editors; The American Chemical Society: Washington , DC, 1986.

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