Bug report

Bug summary

Calling ax.axhline(0) after setting the y-scale to log rescales the y-axis to make the horizontal line appear at 1e-300.

Code for reproduction

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.scatter([1], [1])
ax.set_yscale('log')
ax.axhline(0)
plt.show()

Actual outcome

with the warning

/Users/dstansby/matplotlib/lib/matplotlib/axes/_base.py:3208: UserWarning: Attempted to set non-positive ylimits for log-scale axis; invalid limits will be ignored.
  'Attempted to set non-positive ylimits for log-scale axis; '

Expected outcome
I would expect the horizontal line to be ignored, and axis limits to remain as they were originally.

Matplotlib version

• Operating System: OSX 10.12.5
• Matplotlib Version: master branch installed using pip
• Python Version: 3.6.1