Sideaxis

Sideaxis is a term used in data visualization to describe a secondary axis placed along the side of a chart to display an additional data series with its own scale. It is not a standardized mathematical concept, but a practical feature in many plotting and charting tools. A side axis typically runs parallel to a primary axis (commonly the left vertical Y-axis) and allows readers to compare data sets that use different units or ranges within the same visual frame.

Common characteristics and usage

- Placement: Most often on the left or right edge of a plot as a vertical axis; some systems support a secondary axis on the top or bottom.

- Purpose: To present a second data series or metric that would be hard to compare on the same scale as the primary axis.

- Relationship: The side axis is linked to the data through a scale mapping, which must be clearly described to avoid misinterpretation.

- Design considerations: Use distinct colors, labels, and tick marks; provide a legend; ensure axis ranges are chosen to reflect meaningful comparisons and minimize visual distortion.

Limitations and alternatives

- Side axes can mislead if the scales are not properly synchronized or if there is a large disparity in units. The dual-axis approach requires careful labeling and justification.

- Alternatives include normalizing data, using a single axis with unit conversions, or employing small multiples or faceted charts to compare datasets without a secondary axis.

Related concepts include secondary axis, twin axis, and dual-axis charts, which are common terms for similar ideas in various plotting libraries and software.