Only recommend diagnostic in case a result trajectory is missing

As decided at phone meeting: modelica#3046 (comment)

Reorganizing slightly to avoid paragraph with just a single sentence.

chapters/annotations.tex

@@ -191,16 +191,15 @@ \subsubsection{Plot Curves}\label{plot-curves}
 The mandatory \lstinline!x! and \lstinline!y! expressions are restricted to be result references in the form of \lstinline[language=grammar]!result-reference! in the grammar (\cref{expressions1}), referring to a scalar variable (or a derivative thereof) or \lstinline!time!.
 It is an error if \lstinline!x! or \lstinline!y! does not designate a scalar result.
 If \lstinline!x! or \lstinline!y! is a derivative, \lstinline!der(v, $n$)!, then $n$ must not exceed the maximum amount of differentiation applied to \lstinline!v! in the model.
-When the \lstinline!unit! of an \lstinline!Axis! is non-empty, it is an error if the unit of the corresponding \lstinline!x! or \lstinline!y! expression (i.e., a variable's \lstinline!unit!, or second for \lstinline!time!) is incompatible with the axis unit.
-
-The set of available result variable and derivative trajectories in a simulation result is tool-specific, but it is recommended to include at least the ones being used in figures.
-It is not standardized how a tool should handle missing results for valid result references.
+A diagnostic is recommended in case the simulation result is missing a trajectory for a valid result reference.
 
 \begin{nonnormative}
 While the syntax for referring to a second order derivative is \lstinline!der(v, 2)!, the appearance is left for tools to decide.
 For example, a tool might choose to present this as \lstinline!der(der(v))!.
 \end{nonnormative}
 
+When the \lstinline!unit! of an \lstinline!Axis! is non-empty, it is an error if the unit of the corresponding \lstinline!x! or \lstinline!y! expression (i.e., a variable's \lstinline!unit!, or second for \lstinline!time!) is incompatible with the axis unit.
+
 When \lstinline!legend! is not provided, the tool produces a default based on \lstinline!x! and/or \lstinline!y!.
 Providing the empty string as \lstinline!legend! means that the curve shall be omitted from the plot legend.
 Variable replacements, as described in \cref{variable-replacements}, can be used in \lstinline!legend!.