我遇到了麻烦，让matplotlib找到我知道安装在我的系统上的特定字体，并显示为可用根据matplotlib.font_manager。当我指定在我的图形中使用的特定权重时，我指定的字体不会被使用，链中的其他字体也不会被使用，而是使用链中更下游的字体。
我可以通过

import matplotlib
import matplotlib.font_manager as font_manager

# The backend doesn't matter
#matplotlib.use('agg')

font = {'family': 'sans-serif',
       'sans-serif': ['Open Sans', 'Lucida Grande', 'Avenir', 'Arial', 'sans-serif']}
matplotlib.rc('font', **font)
for font_weight in range(100, 1000, 100):
    matplotlib.rc('font', **{'weight': font_weight})
    print str(font_manager.FontProperties().get_weight()) + ": " + font_manager.FontProperties().get_name()

它产生

100: Open Sans
200: Open Sans
300: Arial
400: Arial
500: Arial
600: Open Sans
700: Open Sans
800: Open Sans
900: Open Sans

这与我在图中得到的结果相对应：对于“正常”，或者对于300到500之间的权重，我得到的不是Open Sans，而是Arial。
这是怎么回事？为什么matplotlib无法找到并使用系统中存在的字体？
FWIW，我可以检查字体管理员知道什么

import matplotlib
import matplotlib.font_manager as font_manager
import glob

the_files = font_manager.findSystemFonts(fontpaths=None)
# Using the_files = font_manager.OSXInstalledFonts() produces the same results
for font_name in ['Open Sans', 'Lucida Grande', 'Avenir', 'Arial']:
    print('')
    print('** ' + font_name)
    print("Names reported by font_manager for fonts in font_manager-found files with '" + font_name + "' in file name:")
    for the_file in the_files:
        if font_name.replace(' ', '').lower() in the_file.replace(' ', '').lower():
            the_props = font_manager.FontProperties(fname=the_file)
            print "\t" + the_props.get_name()

    print('')
    font_in_file_name = font_name.replace(' ', '')
    print("Files found through file system with '" + font_in_file_name + "' in file name:")
    for font_dir in ['/Users/Rax/Library/Fonts/', '/Library/Fonts/', '/System/Library/Fonts/']:
        for file in glob.glob(font_dir + font_in_file_name + '*.*'):
            print("\t" + file)

其确认存在所需字体

** Open Sans
Names reported by font_manager for fonts in font_manager-found files with 'Open Sans' in file name:
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans
    Open Sans

Files found through file system with 'OpenSans' in file name:
    /Users/Rax/Library/Fonts/OpenSans-Bold.ttf
    /Users/Rax/Library/Fonts/OpenSans-BoldItalic.ttf
    /Users/Rax/Library/Fonts/OpenSans-ExtraBold.ttf
    /Users/Rax/Library/Fonts/OpenSans-ExtraBoldItalic.ttf
    /Users/Rax/Library/Fonts/OpenSans-Italic.ttf
    /Users/Rax/Library/Fonts/OpenSans-Light.ttf
    /Users/Rax/Library/Fonts/OpenSans-LightItalic.ttf
    /Users/Rax/Library/Fonts/OpenSans-Regular.ttf
    /Users/Rax/Library/Fonts/OpenSans-Semibold.ttf
    /Users/Rax/Library/Fonts/OpenSans-SemiboldItalic.ttf

** Lucida Grande
Names reported by font_manager for fonts in font_manager-found files with 'Lucida Grande' in file name:

Files found through file system with 'LucidaGrande' in file name:
    /System/Library/Fonts/LucidaGrande.ttc

** Avenir
Names reported by font_manager for fonts in font_manager-found files with 'Avenir' in file name:

Files found through file system with 'Avenir' in file name:
    /System/Library/Fonts/Avenir Next Condensed.ttc
    /System/Library/Fonts/Avenir Next.ttc
    /System/Library/Fonts/Avenir.ttc

** Arial
Names reported by font_manager for fonts in font_manager-found files with 'Arial' in file name:
    Arial
    Arial Black
    Arial
    Arial
    Arial Narrow
    Arial Narrow
    Arial Narrow
    Arial Rounded MT Bold
    Arial
    Arial Unicode MS
    Arial Narrow

Files found through file system with 'Arial' in file name:
    /Library/Fonts/Arial Black.ttf
    /Library/Fonts/Arial Bold Italic.ttf
    /Library/Fonts/Arial Bold.ttf
    /Library/Fonts/Arial Italic.ttf
    /Library/Fonts/Arial Narrow Bold Italic.ttf
    /Library/Fonts/Arial Narrow Bold.ttf
    /Library/Fonts/Arial Narrow Italic.ttf
    /Library/Fonts/Arial Narrow.ttf
    /Library/Fonts/Arial Rounded Bold.ttf
    /Library/Fonts/Arial Unicode.ttf
    /Library/Fonts/Arial.ttf
    /Library/Fonts/ArialHB.ttc

我运行的是OSX 10.9.1，正如上面的代码所指出的，使用OSXInstalledFonts而不是findSystemFonts会产生相同的结果。