gridline labels repeated on sides of a limited view

[evas-ssec]

Description

The left and right gridline labels for a geostationary projection are duplicated and misplaced.

Code to reproduce

import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
import cartopy.crs as crs

# This geostationary projection matches that of GOES 17.
proj_object = crs.Geostationary(central_longitude=-137.0, satellite_height=35786023.0, sweep_axis='x', )
bounding_axes = [-2505021.8, 2505021.8, 1583173.8, 4589200.0]

fig = plt.figure( )
axes = fig.add_subplot(111, projection=proj_object)
axes.set_extent(bounding_axes, crs=proj_object)

# draw some lat/lon gridlines
gl_obj = axes.gridlines(draw_labels=True,)

fig.savefig("./gridtest.png", )
plt.close(fig)

Traceback

(No error message is generated, only the faulty image output.)

Full environment definition

python 3.6.10 ### Operating system MacOS 10.15.7 ### Cartopy version cartopy 0.18.0 ### conda list ``` # packages in environment at /opt/anaconda3/envs/aitf_ql: # # Name Version Build Channel _anaconda_depends 2020.02 py36_0 aitf-ql CSPP.ABI.Quicklooks.v0.7 dev_0 alabaster 0.7.12 py_0 conda-forge anaconda custom py36_1 anaconda-client 1.7.2 py_0 conda-forge anaconda-project 0.8.3 py_0 conda-forge applaunchservices 0.2.1 py_0 conda-forge appnope 0.1.0 py36h9f0ad1d_1001 conda-forge appscript 1.1.0 py36h37b9a7d_1 conda-forge argh 0.26.2 py36_1001 conda-forge asn1crypto 1.3.0 py36h9f0ad1d_1 conda-forge astroid 2.4.1 py36h9f0ad1d_0 conda-forge astropy 4.0.1.post1 py36h37b9a7d_0 conda-forge atomicwrites 1.4.0 pyh9f0ad1d_0 conda-forge attrs 19.3.0 py_0 conda-forge autopep8 1.5.2 pyh9f0ad1d_0 conda-forge babel 2.8.0 py_0 conda-forge backcall 0.1.0 py_0 conda-forge backports 1.0 py_2 conda-forge backports.shutil_get_terminal_size 1.0.0 py_3 conda-forge beautifulsoup4 4.9.1 py36h9f0ad1d_0 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1.2.0 py_0 conda-forge xlsxwriter 1.2.9 pyh9f0ad1d_0 conda-forge xlwings 0.19.4 py36h9f0ad1d_0 conda-forge xlwt 1.3.0 py_1 conda-forge xz 5.2.5 h0b31af3_0 conda-forge yaml 0.2.5 h0b31af3_0 conda-forge yapf 0.29.0 py_0 conda-forge zeromq 4.3.2 h6de7cb9_2 conda-forge zict 2.0.0 py_0 conda-forge zipp 3.1.0 py_0 conda-forge zlib 1.2.11 h0b31af3_1006 conda-forge zstd 1.4.4 h4b3e974_3 conda-forge ```

[djhoese]

I tried tracking this down, but am coming up empty on a final solution. If you lower the Y extents about ~300-500k meters south then you don't get the same issue and you even get more longitude lines. With the original code I went into the cartopy Gridliner source code with pycharm's debugger and as far as I can tell there are indeed duplicate Label objects in the list. There are other Labels that are duplicated (they have the same text and are on the same side of the axes) but one of the duplicates is always set to invisible so it isn't a problem. I'm having trouble following the low-level logic that generates these Labels from the list of ticks. And to be clear, the list of ticks to generate is as expected (ex. [15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, 50.0]) and does not include duplicates.

It has to happen somewhere in this logic which leads to the generation of "heads" and "tails" and then from there the generation of the labels:

https://github.com/SciTools/cartopy/blob/591fb5450e11b42b6de1cebe4f240112f915bd52/lib/cartopy/mpl/gridliner.py#L817-L830

Given this is the geostationary projection and includes points outside of the lon/lat space, my guess is cartopy is failing to check the bounds/overlaps/intersections between the graticule lines and the map bounding box.