Make Axis.unit default to "" (without yet specifying what the default…

… means)

chapters/annotations.tex

@@ -162,15 +162,14 @@ \subsubsection{Axis Properties}\label{axis-properties}
 record Axis
   Real min "Axis lower bound";
   Real max "Axis upper bound";
-  String unit "Unit of min and max";
+  String unit = "" "Unit of min and max";
   String label "Axis label";
 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! unspecified.
+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.
 
-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 not specified.
+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
 \cref{variable-replacements}, can be used in the \lstinline!label! of \lstinline!Axis!  The Modelica tool is responsible for showing the unit used for values at the axis tick marks, so the axis
@@ -189,9 +188,8 @@ \subsubsection{Plot Curves}\label{plot-curves}
 The mandatory \lstinline!x! and \lstinline!y! expressions are restricted to be
 component references refering to a scalar variable or \lstinline!time!. It
 is an error if \lstinline!x! or \lstinline!y! does not designate a scalar variable.
-It is an error if the unit of an expression (i.e., a variable's \lstinline!unit!,
-or second for \lstinline!time!) is incompatible with the \lstinline!unit! of the
-corresponding axis.
+When the \lstinline!unit! of an \lstinline!Axis! is non-empty, it is an error if the unit of the corresponding \lstinline!Curve! expression (i.e., a variable's \lstinline!unit!,
+or second for \lstinline!time!) is incompatible with the axis unit.
 
 When \lstinline!legend! isn't provided, the tool produces a default based on
 \lstinline!x! and/or \lstinline!y!.  Providing the empty string as