don't write y=0 to 2D plot

pub/lib/plot.cpp

@@ -553,9 +553,10 @@ namespace {
             const CSpec* s = fd->VS(j);
             vector<double> xlim = triv::histogram_limits(s->x);
             for (size_t i=0; i<s->size(); ++i)
-                plot->ps_line(
-                    str(format("%13.7g wx %13.7g wx  %13.7g wy %13.7g wy  %13.7g wh t2d")
-                        % xlim[i] % xlim[i+1] % zlim[j] % zlim[j+1] % s->y[i] ) );
+                if (s->y[i])
+                    plot->ps_line(
+                        str(format("%13.7g wx %13.7g wx  %13.7g wy %13.7g wy  %13.7g wh t2d")
+                            % xlim[i] % xlim[i+1] % zlim[j] % zlim[j+1] % s->y[i] ) );
             plot->ps_line("");
         }
         plot->close_frame2D(fd->xco.str_ps(), fd->ZCo[0].str_ps(), fd->yco.str_ps());