\section{Layered and dumbbell comparisons}
\begin{demo}{Current and reference values}
\gradcompare[width=65mm,precision=0]{60}{82}

\graddumbbell[width=65mm,palette=categorical,precision=0]{48}{76}

\graddumbbell[min=-50,max=100,width=65mm,
  compare label=delta,precision=0]{-20}{35}

\graddumbbell[width=65mm,precision=0]{NA}{65}
\end{demo}
Layered bars use a wide reference and a centered current layer. The
\key{compare ratio} defaults to .45 and must be strictly between zero and one.
Both lengths start from the common zero; they are not added or divided.

Dumbbells use a hollow reference point and a current point joined by a line.
The default label is current / reference; delta means current minus reference.
A missing endpoint suppresses the connector; delta is missing if either endpoint
is missing. Equal endpoints overlap. Error whiskers refer to the current value.
\key{compare color}, \key{marker size} and \key{stem width} control appearance.
Dedicated comparison and spark commands retain their shape when applying presets.