Never finagle line fits, delete data points that do not fit the curve, or make data points so large that almost any curve would pass through them. Never distort the importance of a trend, tempting as this may be.
Begin at zero for the scales used for the axes of a graph whenever possible; choose these scales carefully and mark them clearly. Sometimes, a valid trend would disappear on a scale with a zero axis, and all the data points would bunch up at the top. In this case, signal readers that the graph's axis is not at zero, either with a statement in the text or with a break in the axis.
If a point represents the mean of a number of observations, indicate the magnitude of the variability by a vertical line centered at each point. State whether standard error (SE) or standard deviation (SD) is used and specify number of observations or sample sizes. Again, learning the capabilities of your graphing software ultimately will simplify the task. Most programs have the ability to insert such information automatically with a few keystrokes.
When two or more graphs or other figures are to be compared, draw them to the same scale. Then, if possible, group them into a single illustration. To minimize reduction during printing, place them one above the other rather than side by side.
Remember the limitations of your data. The extrapolation of a line or a curve beyond the points shown on a graph may mislead both the writer and the reader.
As Winston Churchill is said to have remarked in another context, "It is wise to look ahead but foolish to look further than you can see."
Image-based illustrations document discoveries. For most people, this is the first category that comes to mind when they hear that a paper has "figures." The photograph of a new organism, the print of gel banding patterns, the radiographic image of a patient's bone deformity ... all would belong here, and in traditional publishing, all would be submitted as a photographic copy. Happily, electronic copy is now widely accepted, so all such illustrations usually can be scanned and submitted as digital files.
When it comes to showing exactly what something looks like, nothing beats the realism of a photograph. At the same time, the realism of a photograph can be disadvantageous. It may include clutter, extraneous information, or components you would rather not show. Happily, the ease by which digital images may be altered makes it possible to fix such distracting components (and almost inexcusable not to do so).
Photographs also will usually only show surfaces, not the components or interior parts that a line drawing or diagrammatic exploded view can provide. Sometimes a traditional drawing is still a better choice.
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