Plot the difference as its own series when differences are the message

Plot the difference itself when the message is how far apart two time series are. For example, add a separate line of pairwise differences on its own common scale instead of asking readers to infer the difference from the changing gap between two curves.

Readers do not naturally recover a clean difference pattern from the space between two curves. They tend to see whichever curves get closest in different regions, while a direct difference plot shows the actual rise and fall of the difference over time.

Mechanism: A direct difference series converts the task into reading values from one common scale. A curve-difference display leaves the reader to judge distances between moving lines, which obscures the intended behavior.

Evidence: The paper shows that curve-difference charts fail to convey even the gross qualitative behavior of the differences, while a direct Cartesian graph of the differences reveals the rise and descent clearly. It recommends plotting the differences on their own graph when differences are to be conveyed (Cleveland & McGill, 1984).

• User Goal: See when the difference between two series grows, shrinks, or changes sign.
• Task: Compare two quantitative series across ordered time.
• Data: Two time-ordered quantitative series with an interpretable point-by-point difference.
• Chart Setting: A two-line or curve-difference display is being considered or already exists.
• Audience: Readers tracking changes in the difference over time.
• Success Criterion: The behavior of the difference is directly visible.

Break it when: The message is the individual levels of the two original series rather than their difference. Why: The source recommends direct difference plots specifically when differences are the quantity to be conveyed.

Sacrifice: The difference gets its own plot or extra series instead of living only as a gap between lines. Risk: If you keep only the gap display, readers may judge whichever curves are nearest in each region instead of the intended difference. Mitigation: Show the original series and the direct difference plot together when both messages matter.

Mistake: Expect readers to read the changing gap between two curves as if it were a direct series. Why it fails: The eye tends to judge minimum spacing in different regions rather than the intended vertical difference pattern.

Failure Sign: Readers describe where the curves look close or far apart but cannot describe the actual difference trend. Quick Check: Ask where the difference grows and shrinks in the current gap display and in a direct difference plot of the same data. Stronger Test: If the gap display does not preserve even the gross behavior of the differences, add the direct difference plot.

• Compute the point-by-point difference between the two series.
• Plot that difference as its own line on a common vertical scale.
• If readers also need the original series, show them in a companion line chart rather than relying on the gap alone.