First try at defining the meaning of empty Axis.unit

chapters/annotations.tex

@@ -169,6 +169,9 @@ \subsubsection{Axis Properties}\label{axis-properties}
 
 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.
 
+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.
+
 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.
 
 When an axis label isn't provided, the tool produces a default label.  Providing the empty string as axis label means that no label should be shown.  Variable replacements, as described in