diff --git a/plot.c b/plot.c
index 498aa88..9fa1c07 100644
--- a/plot.c
+++ b/plot.c
@@ -787,48 +787,29 @@ invalidateRect(int x0, int y0, int x1, int y1){
       mark_map(x, y);
 }
 
+#define SWAP(x,y) {int t=x;x=y;y=t;}
+
 static void
 mark_cells_from_index(void)
 {
-  int t;
+  int t, i;
   /* mark cells between each neighber points */
   for (t = 0; t < TRACES_MAX; t++) {
     if (!trace[t].enabled)
       continue;
-    int x0 = CELL_X(trace_index[t][0]);
-    int y0 = CELL_Y(trace_index[t][0]);
-    int m0 = x0 / CELLWIDTH;
-    int n0 = y0 / CELLHEIGHT;
-    int i;
-    mark_map(m0, n0);
+    int m0 = CELL_X(trace_index[t][0]) / CELLWIDTH;
+    int n0 = CELL_Y(trace_index[t][0]) / CELLHEIGHT;
+    markmap[current_mappage][n0] |= 1<<m0;
     for (i = 1; i < sweep_points; i++) {
-      int x1 = CELL_X(trace_index[t][i]);
-      int y1 = CELL_Y(trace_index[t][i]);
-      int m1 = x1 / CELLWIDTH;
-      int n1 = y1 / CELLHEIGHT;
-      while (m0 != m1 || n0 != n1) {
-        if (m0 == m1) {
-          if (n0 < n1) n0++; else n0--;
-        } else if (n0 == n1) {
-          if (m0 < m1) m0++; else m0--;
-        } else {
-          int x = ((m0 < m1) ? (m0 + 1) : m0) * CELLWIDTH;
-          int y = ((n0 < n1) ? (n0 + 1) : n0) * CELLHEIGHT;
-          int sgn = (n0 < n1) ? 1 : -1;
-          if (sgn*(y-y0)*(x1-x0) < sgn*(x-x0)*(y1-y0)) {
-            if (m0 < m1) m0++;
-            else m0--;
-          } else {
-            if (n0 < n1) n0++;
-            else n0--;
-          }
-        }
-        mark_map(m0, n0);
-      }
-      x0 = x1;
-      y0 = y1;
-      m0 = m1;
-      n0 = n1;
+      int m1 = CELL_X(trace_index[t][i]) / CELLWIDTH;
+      int n1 = CELL_Y(trace_index[t][i]) / CELLHEIGHT;
+      if (m0 == m1 && n0 == n1)
+        continue;
+      int x0 = m0; int x1 = m1; if (x0>x1) SWAP(x0, x1); m0=m1;
+      int y0 = n0; int y1 = n1; if (y0>y1) SWAP(y0, y1); n0=n1;
+      for (; y0<=y1; y0++)
+        for(int j=x0;j<=x1;j++)
+          markmap[current_mappage][y0]|= 1<<x0;
     }
   }
 }
@@ -840,7 +821,6 @@ markmap_upperarea(void)
   invalidateRect(0, 0, AREA_WIDTH_NORMAL, 31);
 }
 
-#define SWAP(x,y) {int t=x;x=y;y=t;}
 //
 // in most cases _compute_outcode clip calculation not give render line speedup
 //