Discontinuous or exponential scales are sometimes used on axes when the range of data values is very wide – that is, very small data values need to be compared with very large data values. A discontinuous axis skips a number of intervals at some point along the axis before continuing. An exponential axis has unevenly spaced intervals, becoming smaller with distance away from the origin. These axis scales are not readily understood by most readers and may give a distorted visual impression or, worse, misrepresent the message of the data.
Find another way to depict the values so that their full scale and the contrast between them is clear – perhaps as 2 different graphs, or a combination of an overall graph and a zoomed-in graph of the critical portion that you want the reader to notice.
Axes that do not start at zero can also distort a reader’s perception of the information; however, they can be useful when the variation between data points occupies only a small range at large values. Make sure the axis labels are clear and, if comparing multiple graphs of similar datasets, ensure that the scale and divisions are consistent, to allow accurate comparison.
Caution! Double or dual-scaled axes in graphs should be avoided because many readers find them difficult to interpret. These graphs typically display a set of data values for 2 different measures on right and left y axes (eg the concentration of methane in the atmosphere over a 20-year period on the left axis, and concentrations of carbon dioxide in the same period on the right axis).
The most common reason for presenting dual-scaled axes is to show the reader similarities in the pattern of values for both measures. However, dual-scaled axes require readers to override their natural inclination to compare data values across the 2 measures – a comparison that is invalid. Consequently, it is almost always better to plot 2 separate graphs (Few 2008b). Use the same design for these graphs to draw attention to similarities in the patterns of data values across the 2 measures, and place the graphs close together on the page. Consider using labels on only one of the x axes, to further link the graphs in the reader’s mind. That is, remove labels from the top or bottom graph (but keep the axis line and tick marks).
Graphs can contain negative values (below zero). For negative values, the axis extends down (vertical y axis) or left (horizontal x axis) from zero. Zero should be clearly marked on the axis, often with a heavier or darker tick line. If both axes are quantitative and span zero, they should intersect at zero. If only 1 axis is quantitative and spans zero, it is customary to place the category axis at the end of the quantitative axis so that the axis labels do not obscure the data. An exception is graphs displaying deviation data, where the goal of the graph is to highlight differences between recorded measurements and some meaningful baseline. This baseline is often represented by the x axis, which intersects the y axis at a zero point that sits approximately halfway up the y axis.
