matplotlib UserWarning When log axis is used in some cases
not sure if this issue is still open, but I had a similar problem and updating seaborn fixed the issue for me. For you it might be matplotlib, depending which package you used for creating the graph.
Basically I am running some optimisation algorithms that I have created using Numpy and I want to plot the log of the error against the number of iterations. Having done this with linear regression and having had no issues, it is very strange that I seem to get issues when doing the exact same thing with logistic regression. I get the following "warning":  

/usr/lib64/python2.6/site-packages/matplotlib/axis.py:1004:
  UserWarning: Unable to find pixel distance along axis for interval
  padding; assuming no interval padding needed.   warnings.warn("Unable
  to find pixel distance along axis for interval padding; assuming no
  interval padding needed.")

However, when I don't use a log axis for the y axis, I don't get the error either. All the elements of the array that I am using are also positive, so it shouldn't have anything to do with taking the log of a non-positive number.
Has anyone ever encountered this before? Does anyone know what it may be referring to?
Thanks