Applying a similar fix to r8873 which seemed to only have been applied to the development branch.

WeatherGod · WeatherGod · commit ea2292e02d8f · 2011-01-05T17:44:17.000Z
This fixes a math domain error when using log scales.

svn path=/branches/v1_0_maint/; revision=8891
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1194,7 +1194,7 @@ def is_decade(x, base=10):
         return False
     if x == 0.0:
         return True
-    lx = math.log(x)/math.log(base)
+    lx = math.log(abs(x))/math.log(base)
     return is_close_to_int(lx)
 
 def is_close_to_int(x):
