Flawed PDF save when using linewidth=0 in drawparallels() and drawmeridians()

Hi all,

I noticed that when saving a basemap plot to a PDF file, the file is flawed when the code has
drawparallels(..., linewidth=0) or drawmerdians(..., linewidth=0) in it.

A minimal working example:

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

figure=plt.figure(figsize=(14,7),dpi=100)
for ii in range(4):
    axii=figure.add_subplot(2,2,ii+1)
    m = Basemap(projection='cyl',
            llcrnrlat=40,urcrnrlat=47,\
            llcrnrlon=-72,urcrnrlon=-67,resolution='l',
            ax=axii, fix_aspect=False)
    m.drawcoastlines()

    lat_ticks=np.arange(41, 47)
    lon_ticks=np.arange(-72, -67)
    m.drawparallels(lat_ticks,labels=[1,0,0,0],linewidth=0)
    m.drawmeridians(lon_ticks,labels=[0,0,0,1],linewidth=0)

figure.savefig('test.pdf')

The PDF won't display correctly in some PDF viewers, for instance Xreader (v.2.4.4) or Adobe reader. But works fine in others, like Okular or Foxit reader. When compiling a LaTeX document using pdflatex, the page containing that figure won't display properly, again it is PDF viewer dependent.

Here is a screenshot of the PDF file opened in Xreader:

Here is the same file opened in Okular:

The reason for setting linewidth=0 is to remove those grid lines, one relevant questions on Stackoverflow.

One workaround I found is to use zorder=-2 to hide those grid lines while avoiding creating corrupted PDF files.

Note that I've tried a few different matplotlib backends, including Qt5Agg, TkAgg, Agg, and the results are all the same.

Some specs and package versions:
OS: Manjaro Linux
Python = 2.7 or 3.7, in an anaconda env
basemap = 1.2.0
matplotlib = 3.1.3 (for py3) and 1.2.0 (for py2)

Beginne damit, das minimale Beispiel auszuführen und die an der PDF-Ausgabe beteiligten Einstiegspunkte drawparallels() und drawmeridians() zu untersuchen. Vergleiche die erzeugte PDF in den betroffenen Viewern und bestätige, dass die Verwendung von linewidth=0 nicht mehr zu einer fehlerhaften Darstellung führt, während die Gitterlinien weiterhin ausgeblendet bleiben.