Fix: prevent divide-by-zero in AxisItem

CHANGELOG

@@ -38,7 +38,7 @@ pyqtgraph-0.9.9  [unreleased]
     - fixed PlotCurveItem generating exceptions when data has length=0
     - fixed ImageView.setImage only working once
     - PolyLineROI.setPen() now changes the pen of its segments as well
-
+    - Prevent divide-by-zero in AxisItem
 
 pyqtgraph-0.9.8  2013-11-24
 

pyqtgraph/graphicsItems/AxisItem.py

@@ -458,8 +458,7 @@ class AxisItem(GraphicsWidget):
             return []
         
         ## decide optimal minor tick spacing in pixels (this is just aesthetics)
-        pixelSpacing = size / np.log(size)
-        optimalTickCount = max(2., size / pixelSpacing)
+        optimalTickCount = max(2., np.log(size))
         
         ## optimal minor tick spacing 
         optimalSpacing = dif / optimalTickCount