\section{Stacked contributions and segment labels}
\begin{demo}{Positive and negative contributions}
\gradstack[min=-60,max=100,width=80mm,height=14pt,
  precision=0,palette=categorical,stack totals=separate,
  stack names={Sales,Materials,Service,Operations},
  segment labels=value,legend=true]{60,-25,20,-15}
\end{demo}
Positive and negative contributions accumulate independently from zero. Within
each side, segment order follows input order. Negative segments require a negative
min. Positive and negative subtotals are checked separately against the scale.
The default total is the algebraic sum; \key{stack totals=separate} shows positive
and negative subtotals. A zero net does not erase nonzero contributions.

Segment labels can be none, value, percent, name, name value or name percent.
The percentage of segment $i$ is
\[
  p_i=\frac{100|v_i|}{\sum_j|v_j|},
\]
where $v_i$ is the original signed value of segment $i$, and $v_j$ is the
original signed value of segment $j$. The sum runs over all input segments,
including contributions clipped by the display range. These percentages
represent shares of absolute activity, not shares of a potentially zero net.
All-zero stacks do not calculate percentages. Stacks are not normalized to full
width. Missing segments, empty lists, quality thresholds and error whiskers are rejected.

Provide one stack name per segment for named labels or automatic legends.
Small labels move outside with leader lines; automatic black/white text follows
the fill. Zero segments retain their palette and legend positions. Use a shared
\key{\gradbarslegend} outside a table to avoid repeating the legend for every row.