\section{Continuous and shared legends}
Load \key{legend} in the preamble; it depends on Heat. Unlike category swatches,
a continuous legend describes a numeric scale and uses the same color interpolation.
\begin{demo}{Two values sharing a frozen scale and legend}
\begingroup
\gradscale{legend-score}{min=-10,max=30}
\gradlegenddefine{legend-score}{scale group=legend-score,
  heat palette=purple-teal,
  heat center=0,legend title={Score}}
\gradheat[legend group=legend-score]{-5}
\gradheat[legend group=legend-score]{20}
\par\medskip
\gradlegenduse[width=60mm,height=3mm,legend missing=false]{legend-score}
\endgroup
\end{demo}
\key{\gradheatlegend} accepts ordinary graphic keys plus \key{legend title},
\key{legend ticks}.\par
\key{legend missing=false} hides the missing-value item and its automatic description;
\key{true} is the default. Both legend commands accept this switch.\par
Set \key{legend orientation} to \key{horizontal} or \key{vertical}.
Width is ramp length; height is thickness. Endpoints and an interior neutral
center are default ticks; custom ticks must increase within the range. The smooth
ramp uses 256 vector strips. Increase precision or spacing if tick text overlaps.
\key{\gradlegenddefine} freezes scale, direction, unit and color roles in the
current group. Use \key{legend group=name} in standalone graphics or column
options. Unknown names, duplicate declarations and contradictory scale/color/unit
settings are errors. \key{\gradlegenduse} only accepts layout, tick formatting,
font and accessibility overrides, leaving subsequent graphics unchanged.
\begin{demo}{Reference symbols have different statistical meanings}
\gradreferencelegend{target/Declared target,
  reference/Reference value,band/Reference range,
  error/Error distances,box/Quartiles and observed whiskers}
\end{demo}
Reference descriptions declare their actual meaning; neither error distances
nor box whiskers automatically denote confidence intervals.
For shared category or reference legends, set \key{legend kind=categorical|reference}
and \key{legend names}. Repeated decoration may use \key{accessibility=artifact}.


\subsection{Built-in Heat palettes}
One \key{heat palette=name} selects endpoint colors, a white center and the interpolation mode.
Sequential palettes \key{blue}, \key{teal}, \key{purple} and \key{orange} run from white to that color.
Diverging palettes \key{purple-teal}, \key{blue-red}, \key{orange-blue} and \key{brown-teal}
run from the first color through white to the second; \key{heat center} still sets the numeric center.
These are color shortcuts, not accessibility certifications; existing accessible palettes remain available.
Explicit low, center and high colors placed after the preset still override it.
\begin{demo}{Built-in continuous palettes}
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=blue,legend title={blue},legend missing=false]\quad
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=teal,legend title={teal},legend missing=false]\par\medskip
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=purple,legend title={purple},legend missing=false]\quad
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=orange,legend title={orange},legend missing=false]\par\medskip
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=purple-teal,legend title={purple-teal},legend missing=false]\quad
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=blue-red,legend title={blue-red},legend missing=false]\par\medskip
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=orange-blue,legend title={orange-blue},legend missing=false]\quad
\gradheatlegend[width=45mm,height=2mm,min=-10,max=30,heat center=0,
  heat palette=brown-teal,legend title={brown-teal},legend missing=false]
\end{demo}