don't try to plot if gp_fnames is empty

pub/src/dualplot.cpp

@@ -460,11 +460,11 @@ void CPlot::addScan( bool as_line, int style_no,
 
 void CPlot::showScans()
 {
-    string cmd = "plot " + 
-        str( format( "[%12.8g:%12.8g] [%12.8g:%12.8g] " )
-                     % X.inf % X.sup % Y.inf % Y.sup ) +
-        gp_fnames;
-    gp_write( cmd );
+    if ( gp_fnames!="" )
+        gp_write( "plot " + 
+                  str( format( "[%12.8g:%12.8g] [%12.8g:%12.8g] " )
+                       % X.inf % X.sup % Y.inf % Y.sup ) +
+                  gp_fnames );
 }
 
 //! Add documentation lines to postscript output.