diff --git a/src/OpenMesh/Tools/Utils/Gnuplot.cc b/src/OpenMesh/Tools/Utils/Gnuplot.cc
index 88e91607..bf852619 100644
--- a/src/OpenMesh/Tools/Utils/Gnuplot.cc
+++ b/src/OpenMesh/Tools/Utils/Gnuplot.cc
@@ -126,7 +126,7 @@ Gnuplot::Gnuplot(const string &style)
 Gnuplot::Gnuplot(const string &title,
                  const string &style,
                  const string &labelx,  const string &labely,
-                 vector<double> x, vector<double> y)
+                 const vector<double>& x, const vector<double>& y)
 {
   init();
 		
@@ -157,7 +157,7 @@ Gnuplot::Gnuplot(const string &title,
 
 Gnuplot::Gnuplot(const string &title,  const string &style,
                  const string &labelx, const string &labely,
-                 vector<double> x)
+                 const vector<double>& x)
 {
   init();
   
@@ -349,7 +349,7 @@ void Gnuplot::plot_equation(const string &equation, const string &title)
 
 // ----------------------------------------------------------------------------
 
-void Gnuplot::plot_x(vector<double> d, const string &title)
+void Gnuplot::plot_x(const vector<double>& d, const string &title)
 {
   ofstream tmp;
   ostringstream cmdstr;
@@ -418,7 +418,7 @@ void Gnuplot::plot_x(vector<double> d, const string &title)
 
 // ----------------------------------------------------------------------------
     
-void Gnuplot::plot_xy(vector<double> x, vector<double> y, const string &title)
+void Gnuplot::plot_xy(const vector<double>& x, const vector<double>& y, const string &title)
 {
   ofstream tmp;
   ostringstream cmdstr;
diff --git a/src/OpenMesh/Tools/Utils/Gnuplot.hh b/src/OpenMesh/Tools/Utils/Gnuplot.hh
index faf2e50c..0f6599e4 100644
--- a/src/OpenMesh/Tools/Utils/Gnuplot.hh
+++ b/src/OpenMesh/Tools/Utils/Gnuplot.hh
@@ -120,14 +120,14 @@ public:
           const string & _style,
           const string & _xlabel,
           const string & _ylabel,
-          vector<double> _x, vector<double> _y);
+          const vector<double>& _x, const vector<double>& _y);
   
   /// Constructor calling plot_x().
   Gnuplot(const string &_title,
           const string &_style,
           const string &_xlabel,
           const string &_ylabel,
-          vector<double> _x);
+          const vector<double>& _x);
   //@}
 
   ~Gnuplot();
@@ -146,10 +146,10 @@ public:
   //@{
 
   /// Plot a single vector
-  void plot_x(vector<double> _x, const string &_title);
+  void plot_x(const vector<double>& _x, const string &_title);
   
   /// Plot x,y pairs
-  void plot_xy(vector<double> _x, vector<double> _y, const string  &_title);
+  void plot_xy(const vector<double>& _x, const vector<double>& _y, const string  &_title);
   
   /// Plot an equation of the form: y = ax + b
   /// You supply a and b