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Axis Reference Lines

Axis reference (const) lines are used when you want to visually emphasize that some axis value is significant to the data being presented.

Axis reference lines with text annotations at the X and Y axes.

Axis reference lines in 3D.

Use constant lines for key references like critical thresholds, target values, or averages with constant lines to guide interpretation.
Use distinct styling: Style constant lines differently from gridlines—e.g., dashed lines, different colors, or increased thickness.
Limit the number of constant lines: Use only a few to avoid visual clutter; too many reference lines can overwhelm the chart.
Align with context: Place constant lines where they add value—don’t force them if they don’t highlight a meaningful insight.
Use tooltips for interactivity: In interactive charts, enhance constant lines with tooltips or hover info for additional context.
Support multiple axes: If using dual axes, clearly associate constant lines with the correct axis via color or label alignment.