Fix problem with text plots of arrays of all missing data (such as when there's no rain in the mesonet example.
Fix problem with text plots of arrays of all missing data (such as when there's no rain in the mesonet example.

git-svn-id: acf0ef94bfce630b1a882387fc03ab8593ec6522@205 150532fb-1d5b-0410-a8ab-efec50f980d4

ahaberlie/MetPy,dopplershift/MetPy,deeplycloudy/MetPy,Unidata/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,jrleeman/MetPy,Unidata/MetPy,dopplershift/MetPy,ahaberlie/MetPy,ahill818/MetPy