Incorporate suggestions from #2578

Clarify that behavior is unspecified for:
 - Curves where x or y does not designate a variable
 - Unknown units or incompatible with the plotted variable

chapters/annotations.tex

@@ -165,7 +165,9 @@ \subsubsection{Plot Curves}\doublelabel{plot-curves}
 \end{lstlisting}
 
 The mandatory \lstinline!x! and \lstinline!y! expressions are restricted to be
-component references refering to a scalar variable or \lstinline!time!.
+component references refering to a scalar variable or \lstinline!time!. If
+\lstinline!x! or \lstinline!y! does not designate a variable the behavior is
+unspecified.
 
 When \lstinline!legend! isn't provided, the tool produces a default based on
 \lstinline!x! and/or \lstinline!y!.  Providing the empty string as
@@ -187,8 +189,11 @@ \subsubsection{Axis Properties}\doublelabel{axis-properties}
 When an axis bound isn't provided, the tool computes one automatically.
 
 The Modelica tool is responsible for showing the unit used for values at the
-axis tick marks, so the axis \lstinline!label! shall not be used to convey this
-information.
+axis tick marks, so the axis \lstinline!label! shall not contain the unit.
+
+If the specified unit is not supported by the tool, or the unit does not
+represent a display unit of the plotted variable (for example,
+\lstinline!unit="cm"! and variable denotes a mass), the behavior is unspecified.
 
 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