Biomedical Engineering Program Louisiana Tech University

Steven A. Jones Revised March 5, 2016

Checklist for Graphs

You must perform the following functions before you turn in any reports. These actions are easy to do and can immensely improve the professional character of your report (Show Me). Please check off the following and turn this form in with your report. Reports will not be accepted for grading until you have undergone these checks and may be considered as “late” if they must be returned to you for lack of your attention to these details.

  1. Where possible and appropriate, multiple related curves are plotted on the same graph so that they can be readily compared. (How?)
  2. The graph background is white. (How?)
  3. The “Smoothed line” option is turned off. If no theoretical curve or appropriate curve fit, I have not connected the symbols or have connected them with straight lines (How?).
  4. Collected data are represented with symbols, and theoretical curves or digitized waveforms with lines (How?).
  5. Line modes, line thicknesses or symbols distinguish data sets, not colors (How?).
  6. The horizontal gridlines are removed from my plots (How?).
  7. Major tick marks are included on the axes to indicate the positions corresponding to the number labels (How?).
  8. The numbers on the x and y axes are large enough to read (How?).
  9. The scales on the x and y axes follow the 1, 2, 5 rule (How?).
  10. The x and y axes are labeled clearly with the information they represent and the correct units of the data (e.g. Frequency (Hz) or Pressure (dynes/cm2)) (How?).
  11. The y-axis labels (title and numbers) run vertically from the bottom of the plot to the top, rather than horizontally (How?).
  12. Greek letters, other special characters, or superscripts/subscripts are used, when necessary, in an axis title or elsewhere (How?).
  13. The legend for each curve describes only the distinguishing characteristic of the curve (e.g. “With Fibrinogen”, “Without Fibrinogen,” not “Cell Growth With Fibrinogen,” “Cell Growth Without Fibrinogen,” not “first data set,” “second data set,” and certainly not “series 1,” “series 2.”) (How?).
  14. No chart title is used unless multiple graphs appear in a single figure. (How?)
  15. The legend is on the plot area at a location where it does not cover any of the data. (How?)
  16. Where applicable, the legends appear in an order that follows the position of the curve on the plot (i.e., upper curve matches the topmost legend) (How?).
  17. Legends highlight only the aspect of the given curve that distinguish it from the other curves. (Generally, if all legends have a phrase in common, that phrasecan be removed).
  18. Unnecessary borders are removed, including:
  19. The border around the legend.
  20. The border around the plotting area.
  21. The border around the complete plot. (How?)
  22. Where applicable, bars representing standard deviation or standard error of my collected data are included (How?).
  23. Where reasonable, physical units for the axes allow the number labels to have a small number of digits (e.g. 10 GPa instead of 10E9 Pa or 10,000 MPa) (What?).

Contents

Motivation 2

Multiple Curves 3

Chart Title 5

Changing the Plot Background 6

Smoothed Line Option (Never use this option) 6

Symbols and Theoretical Curves 7

Line Modes and Thicknesses 8

Grid Lines 8

Tick Marks 8

Fonts on the Axes 9

One, Two, Five Rule 9

Axis Titles 10

Vertical Text 10

Greek Letters, Superscripts and Subscripts 11

Borders (Get rid of them) 11

Data Legends 12

Legend Position 13

Series Order 14

Standard Deviations 15

Use of a Template 17

Motivation

Graphs are arguably the most powerful tool that you have available in technical writing and technical presentations. When properly used, they can convey a large amount of information to the reader quickly and effectively. Time invested into proper formatting of the graph is worthwhile not only because it makes the plots look better, but also because it makes the data more readily understandable and illustrates the point of your work more effectively. This document provides several basic guidelines for creating effective graphs and illustrates the steps required to make basic formatting changes in Excel.

Figures 1 and 2 are two graphical representations of the same data set. Figure 1 uses the default format of Excel. In Figure 1, additional care has been taken to follow the guidelines. The plot in Figure 2 provides the reader with an immediate understanding of the underlying data.

Figure 1: Graph generated with the Excel default format. / Figure 2: Graph of the same data used in Figure 1, but following the guidelines.

Figure 3 shows a default-style Excel plot in which experimental data are compared to a theoretical curve. Figure 4 presents the same information in a more professional style.

Figure 3: Graph of theory and data generated in Excel default format. / Figure 4: Graph of the same data used in Figure 3 but following the guidelines.

Multiple Curves

In general, multiple data sets that have the same x and y axes should be plotted on the same graph. A plot with multiple data sets (curves) allows the data sets to be easily compared to one another and requires much less manuscript space than multiple graphs. Each curve must have a distinguishing line mode and legend. Excel generally attempts to determine how your data are ordered when you make a plot. For example, if you have 4 columns of data and you highlight all four columns, then go to the Insert ribbon and click the small arrow next to the scatter plot icon in the Charts group (Figure 6) and select the straight line-only style, Excel assumes that you wish to use the data in the first column for x-axis values and to use the other three columns as y-axis values for three separate curves. Sometimes you will need to manually tell Excel which data sets are to be used for x and y values on each curve.

Figure 5: Selection of the straight line-only graph type (highlighted in green).

To change the data you wish to plot, right click on the plot you have created and go to “Select data” to obtain the Select Data Source window shown in (Figure 6). Click on the series of interest to bring up the Edit Series window (Figure 7), and change the information as necessary. The example in Figure 7 shows the information for a worksheet named “Young’s Equation,” where the x-axis values are in Column A from Row 32 to Row 46, and the y-axis values are in Column C from Row 32 to Row 46. The series name (which will be used as the legend for this curve) is “Long Stenosis.”

Figure 6: The Select Data Source window

Figure 7: The Edit Series window

You can click on the “Add” button in the Select Data Source window to create an additional curve. Select the x and y data values as described in the previous paragraph.

Chart Title

Chart titles are not necessary in a document when a figure caption refers only to a single graph. The chart title would be redundant to the figure caption. However, when a figure caption refers to multiple figures (e.g. Figure 12a and Figure 12b), the chart title can be used to distinguish the two subfigures. While the use of multiple curves on a single graph is generally preferred to multiple graphs, multiple graphs may be preferred or necessary under the following circumstances.

  1. The data sets are fundamentally different from one another because either the x-axes or the y-axes are different physical quantities.
  2. The ranges of one of the axes are too different from one another. For example, one data set of force as a function of time ranges from 1 N to 1000 N, while the other data set ranges from 0.1 N to 10 N. If you need the reader to see any detail in the second data set, it may need to be plotted on a different scale. However, you may consider the use of a log axis or the use of two axes (left and right).
  3. The number of curves that are presented on one graph is so large that the curves obscure one another and prevent the reader from visualizing the trends. Here, you must choose between eliminating some of the data sets or showing some of the data sets on a separate graph.

You need a chart title under two circumstances:

  1. Your figure consists of multiple graphs, each of which illustrates the results of a different condition. For this type of figure, each graph should have a title that differentiates it from the other graphs. Do not generate multiple graphs if multiple curves are more appropriate.
  2. Your figure will be part of a PowerPoint presentation or other venue in which figure captions are not used.

To remove a chart title, left click on it and hit the delete key. To create a chart title, left click on the graph and then left click on the + sign that appears to the upper right of the graph. This action brings up a menu of elements that you can add (Figure 8).

Figure 8: The Add Elements menu

You can also click on the graph, click on the Design tab, and use the “Add Chart Element” button in the “Chart Layouts” group at the upper left (Figure 9).

Figure 9: The Add Chart Elements button in the Chart Layouts group.

Changing the Plot Background

The background of your plot should be white or, in a PowerPoint presentation, the same color as the background of your master slide. If the background is a color other than white, left click on the colored area and hit the delete key to change it to white. To change it to any other color, left click on the area, elect “FORMAT” under the “CHART TOOLS” menu that appears, and click on the down-arrow next to “Shape Fill.” Then select the desired color ().

Figure 10: The “CHART TOOLS” menu with the “Shape Fill” button.

Smoothed Line Option (Never use this option)

The “Smoothed Line” option connects data points with a “spline fit” of the data. This feature is almost never desirable in scientific writing. It usually has no scientific relevance to your data and it can mislead the reader. It can also lead to data points on the curve that are physically impossible (e.g. showing negative values for something that must be a positive quantity). To eliminate this option, right click on the data series in question, go to “Format Data Series…,” (or alternatively, double click on the data series) and use the menu that appears on the right side of the screen (Left side of Figure 11). Select the “Fill & Line” icon (the little paint can under “SERIES OPTIONS,” and expand the “LINE” menu. Go to the bottom of the menu and uncheck “Smoothed line.”

Figure 11: The “Format Data Series…” menu with (Left) the ~LINE selected and (Right) the ~MARKER selected.

Symbols and Theoretical Curves

Collected data should generally be presented as discrete symbols, and theoretical curves or curve fits should be presented as lines. Digitized data, such as the time series data that you might download from the oscilloscope, should also be presented as a line. To represent discrete data points with symbols, right click on the data series in question, go to “format data series.” You will obtain the “Format Data Series…” menu that was shown in Figure 11. Select the “Fill & Line” icon (the little paint can under “SERIES OPTIONS,” and expand the “LINE” menu. Select “No line.” Then click on “~Marker” button and click on “Built-In.” Under “Type,” select the shape of the marker that you wish to use. Now click on “Marker Fill,” select “Solid fill,” and use the down-arrow next to the “Color” box to select the desired color (usually black). In some cases, you may need to change the marker line color (the outline of the marker) to match the marker fill. To do so, go down to the “Border” section at the bottom of the menu and select the appropriate color under the “Color” drop down menu (not shown in the figure).

You can distinguish different data sets by different symbols and different fill options. For example, squares vs. diamonds vs. triangles, or filled squares vs. open squares. Be consistent in selecting symbols. For example, if you are plotting x vs. y for two models at three different temperatures, you can make Model 1 always be filled symbols and Model 2 always be unfilled symbols and differentiate the temperatures as boxes, diamonds and triangles.

Line Modes and Thicknesses

Whenever possible, generate plots in black and white rather than color. While colors may look nice, they do not necessarily copy well. While you may believe that it does not matter because you plan to print your final thesis or report in color, consider that some people may have a black and white copy of your work and may miss important aspects of your work. If you are making a plot for a presentation, remember that some colors do not appear at all when displayed on computer projectors, leading you to tell your audience, with great embarrassment, that “It looks fine on my computer screen.” You will then have to explain to them what the data set “would look like” if they could see it.

To change the character of plotted curves, bring up the menu on the left side of Figure 11. Select the various options as appropriate.

Note: If you are plotting a large number of points, the dashed and dotted line mode options will not work correctly. You can make it work correctly if you reduce the number of points in your data set (e.g., keeping only every 5th point). One simple way to do reduce the number of points is to highlight the data points that you want to keep (using CTRL+left-click), then use copy, and then past the results to another location in the spreadsheet.

Grid Lines

By default, Excel plots have horizontal gridlines in them. These lines are distracting and should be removed. To do so, click on one of the lines and hit the Delete key. Alternatively, you can right click on the grid lines and scroll down to “Delete.”