[Matplotlib-users] Font family ['cmb10'] not found

Fedora f15.  What am I missing that causes this?

/usr/lib64/python2.7/site-packages/matplotlib/font_manager.py:1242: UserWarning: 
findfont: Font family ['cmb10'] not found. Falling back to Bitstream Vera Sans
  (prop.get_family(), self.defaultFamily[fontext]))
/usr/lib64/python2.7/site-packages/matplotlib/font_manager.py:1252: UserWarning: 
findfont: Could not match :family=Bitstream Vera 
Sans:style=normal:variant=normal:weight=normal:stretch=normal:size=12. Returning 
/usr/share/fonts/texlive-gfsdidot/GFSDidotBold.otf
  UserWarning)
/usr/lib64/python2.7/site-packages/matplotlib/font_manager.py:1242: UserWarning: 
findfont: Font family ['cmtt10'] not found. Falling back to Bitstream Vera Sans
  (prop.get_family(), self.defaultFamily[fontext]))
/usr/lib64/python2.7/site-packages/matplotlib/font_manager.py:1242: UserWarning: 
findfont: Font family ['cmss10'] not found. Falling back to Bitstream Vera Sans
  (prop.get_family(), self.defaultFamily[fontext]))