A sequential palette maps ordered, usually continuous values to a continuous color ramp. More color represents larger values; less color represents smaller ones. section 4.3.1
The simplest approach keeps one hue and varies saturation or lightness. Varying both makes the scale easier to interpret in black and white and can support colorblind-friendly design when background contrast is sufficient. section 4.3.1
Use a sequential palette when:
- The data has meaningful order, especially continuous values. section 4.3.1
- You want the amount of color to communicate magnitude rather than category. section 4.3.1
- You need stronger separation than a pale single-hue ramp provides; a two-hue gradient can preserve color across the scale and improve visibility. section 4.3.1
A readable ramp
Imagine values running from low to high. Begin with white, gray, or a lightly colored version of one hue, then increase saturation and lightness or darkness toward the high end. If the low values disappear against the background, introduce a second hue while continuing to vary brightness. section 4.3.1
Figure 4.22 A continuous sequential color palette, made in HSL space by selecting hue = 270, varying the saturation from 0% (no color) to 100% (full color), and varying the lightness from 100% (white) to 50% (full color).
The figure shows a sequential palette in HSL space: one hue stays fixed while saturation rises from no color to full color and lightness moves from white toward full color. section 4.3.1
Figure 4.23 A single-hue gradient (on the left) can make it easy to lose the lower data points in the background if that contrast isn’t high enough. To avoid such problems, one can use a two-hue gradient instead (on the right). Notice how much easier it is to see where the higher data values are.
The comparison shows why a two-hue gradient can make higher values easier to see when a single-hue ramp loses lower values against the background. section 4.3.1
