Separate meanings of presence of axis bounds and presence of Axis.unit

chapters/annotations.tex

@@ -167,10 +167,15 @@ \subsubsection{Axis Properties}\label{axis-properties}
 end Axis;
 \end{lstlisting}
 
-When an axis bound isn't provided, the tool computes one automatically.  When \lstinline!min! or \lstinline!max! is specified, it is an error to leave \lstinline!unit! empty.
+When an axis bound isn't provided, the tool computes one automatically.
 
-An empty \lstinline!unit! means that each expression plotted against it uses its own \lstinline!displayUnit!.  It is not required that the different expressions have compatible units; instead a unitless
-axis may be used, with unit information attached to curve legends instead.
+An empty \lstinline!unit! means that the axis is unitless, and each expression plotted against it may use its own unit determined by the tool.  The tool is responsible for conveying the information
+about choice of unit for the different variables, for instance by attaching this information to curve legends.
+
+\begin{nonnormative}
+When \lstinline!unit! is empty, and axis bounds are to be determined automatically, a natural choice of unit could be the variable's \lstinline!displayUnit!.  When axis bounds are specified by the
+user, on the other hand, a tool may choose a unit for the variable such that the range of the variable values (expressed in the chosen unit) fit nicely with the range of the unitless axis.
+\end{nonnormative}
 
 If a tool does not recognize the \lstinline!unit!, it is recommended to issue a warning and treat the \lstinline!unit! as if it was empty.