diff --git a/plz_draw b/plz_draw
index 90e33ad..1ab6ad9 100755
--- a/plz_draw
+++ b/plz_draw
@@ -219,7 +219,9 @@ def draw_voronoi(ctx, geoitems, options):
 
     plzlines = {}
     for (l, p1, p2) in edges:
-        x1 = y1 = x2 = y2 = None
+        # l ist der Index der Original Daten in lines[]
+        # p1 und p2 sind der Index der Endpunkte in points[p2]
+        x1 = y1 = x2 = y2 = None  # Endpunkte der zu zeichnenen Linie
         a, b, c, plz1, plz2 = lines[l]
         if p1 > -1:
             x1, y1 = points[p1]
diff --git a/pygeodb/voronoi.py b/pygeodb/voronoi.py
index e5a849c..41e542f 100644
--- a/pygeodb/voronoi.py
+++ b/pygeodb/voronoi.py
@@ -102,7 +102,7 @@ def usage():
 import math
 import sys
 import getopt
-TOLERANCE = 1e-9
+TOLERANCE = 1e-15
 BIG_FLOAT = 1e38
 
 #------------------------------------------------------------------
@@ -405,11 +405,15 @@ def bisect(s1, s2):
             newedge.a = 1.0
             newedge.b = dy/dx
             newedge.c /= dx
-        else:
+        elif dy:
             # set formula of line, with y fixed to 1
             newedge.b = 1.0
-            newedge.a = dx/(dy+0.000001)
-            newedge.c /= (dy+0.000001)
+            newedge.a = dx/dy
+            newedge.c /= dy
+        else:
+            newedge.b = 1.0
+            newedge.a = dx/(dy+0.0000001)
+            newedge.c /= (dy+0.0000001)
 
         newedge.edgenum = Edge.EDGE_NUM
         Edge.EDGE_NUM += 1