bug fix: plot2D did not clear previous plot

pub/plot/ps_plotter.cpp

@@ -66,6 +66,7 @@ void CPS_Plotter::start_frame2D(
     Y = &_Y;
     ps_snum = 0;
     ps_Doc = "";
+    ps_accu = "";
     ps_accu += ps_coord(X);
     ps_accu += ps_coord(Z);
     ps_accu += ps_coord(Y);