Bug report

Bug summary

When changing the color of major gridlines in a plot, some major grid lines will occasionally have a gray, slightly transparent line placed over it.

Code for reproduction

# Paste your code here
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import numpy as np
import matplotlib.pyplot as plt

aX = np.array([4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24])			# x-axis
aY = np.array([1,4,10,20,35,56,80,104,125,140,146,140,125,104,80,56,35,20,10,4,1])	# y-axis
aY = aY / aY.sum()

plt.plot(aX, aY)														# plot curve and show

plt.xlim((0, 30))
plt.ylim(ymin = 0)
plt.grid(True, 'major', color='k')
plt.minorticks_on()
plt.grid(True, 'minor', 'y')
plt.show()

Actual outcome

Note the major grid line at 0.06 seems to be "shaded over". Additionally, this issue doesn't happen if aY = aY / aY.sum() is commented out.

# If applicable, paste the console output here
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Expected outcome

I would expect that all major grid lines have the color specified by plt.grid(True, 'major', color='k').

Matplotlib version

• Operating system: Windows 7 Professional 64bit
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg
• Python version: 3.6.2
• Jupyter version (if applicable): N/A
• Other libraries: numpy

I installed python from the available .exe file. I installed matplotlib according to the instructions on the website (with pip).