I packaged v0.3.1 for nixpkgs (NixOS/nixpkgs#419858) and in the process discovered that four tests fail with an ImageComparisonFailure and a quite high RMS value:

• test_labels
• test_complex
• test_display_shortest_path
• test_labels_and_colors

For now I disabled those tests without investigating further, but I was wondering whether you see any pattern here or have any hunch why those might be failing on my system.