Only allow dB conversion factors 10 and 20

chapters/annotations.tex

@@ -216,19 +216,19 @@ \subsubsection{Axis Properties}\label{axis-properties}
 \begin{lstlisting}[language=modelica]
 record dB
   extends AxisScale;
-  Integer factor(min = 1);
+  Integer factor "Decibel conversion factor, either 10 or 20";
 end dB;
 \end{lstlisting}%
 \index{dB@\robustinline{dB} (decibel axis scale)}
 
 The mandatory \lstinline!factor! is used to define a tick label conversion according to $y \mapsto \text{\lstinline!factor!}\, \operatorname{log}_{10}(y)$.
 This mapping shall be applied before presenting values in tick labels.
+The only allowed values for \lstinline!factor! are 10 and 20.
 It is recommended that value of \lstinline!factor! is somehow indicated on the axis or its tick labels, for example by presenting a major tick label as $-40\, \mathrm{dB}_{20}$.
 Major axis ticks are preferred at integer converted values, and minor ticks should be placed at evenly distributed converted values (similar to a linear axis scale).
 
 \begin{nonnormative}
-The two values of \lstinline!factor! which are in widespread use are 10 and 20.
-10 is typically used when plotting signal power, and 20 is typically used when plotting signal amplitude.
+The \lstinline!factor! value 10 is typically used when plotting signal power, and 20 is typically used when plotting signal amplitude.
 \end{nonnormative}
 
 \begin{example}