Use scalable graphics for synchronous example

chapters/synchronous.tex

@@ -16,7 +16,7 @@ \subsection{Overview}\label{overview}
 The following small example shows the most important elements:
 \begin{figure}[H]
   \begin{center}
-    \includegraphics[width=7in,height=4in]{plantmodel}
+    \includegraphics{plantmodel}
   \end{center}
   \caption{A continuous plant and a sampled data controller connected together with sample and (zero-order) hold elements.}
 \end{figure}
@@ -26,7 +26,7 @@ \subsection{Overview}\label{overview}
   A periodic clock is defined with \lstinline!Clock(3)!. The argument
   of \lstinline!Clock! defines the sampling interval (for details see \cref{clock-constructors}).
 \item
-  Clocked variables (such as yd, xd, ud) are associated uniquely
+  Clocked variables (such as \lstinline!yd!, \lstinline!xd!, \lstinline!ud!) are associated uniquely
   with a clock and can only be directly accessed when the associated
   clock is active. Since all variables in a clocked equation must belong
   to the same clock, clocking errors can be detected at compile time. If
@@ -56,16 +56,16 @@ \subsection{Overview}\label{overview}
   The \lstinline!when!-clause in the controller could also be removed
   and the controller could just be defined by the equations:
 \begin{lstlisting}[language=modelica]
-// discrete controller
-E*xd = A*previous(xd) + B*yd;
-ud = C*previous(xd) + D*yd;
+/* Discrete controller */
+E * xd = A * previous(xd) + B * yd;
+    ud = C * previous(xd) + D * yd;
 \end{lstlisting}
 \item
   \lstinline!previous(xd)! returns the value of \lstinline!xd! at
   the previous clock tick. At the first sample instant, the start value
   of \lstinline!xd! is returned.
 \item
-  A discrete-time signal (such as ud) is converted to a continuous-time signal with \lstinline!hold!.
+  A discrete-time signal (such as \lstinline!ud!) is converted to a continuous-time signal with \lstinline!hold!.
 \item
   If a variable belongs to a particular clock, then all other
   equations where this variable is used, with the exception of as