Fix partial link attribute plotting for 'gpd' backend

[kbonney]

Summary

This PR extends work started in #563 to links. When a partial link attribute is provided, the other links are still shown in black rather than not being shown at all (the desired behavior). This has been fixed here.

Tests and Documentation

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[coveralls]

coverage: 85.984%. remained the same — kbonney:fix-gpd-links into USEPA:main