diff --git a/plot.c b/plot.c
index 9996d91..2475a87 100644
--- a/plot.c
+++ b/plot.c
@@ -811,7 +811,7 @@ mark_cells_from_index(void)
       int y0 = n0; int y1 = n1; if (y0>y1) SWAP(y0, y1); n0=n1;
       for (; y0<=y1; y0++)
         for(j=x0; j<=x1; j++)
-         map[y0]|= 1<<x0;
+         map[y0]|= 1<<j;
     }
   }
 }