Makes informative labels if variable metadata (attrs) follows
        CF conventions.

def label_from_attrs(da, extra=''):
    ''' Makes informative labels if variable metadata (attrs) follows
        CF conventions. '''

    if da.attrs.get('long_name'):
        name = da.attrs['long_name']
    elif da.attrs.get('standard_name'):
        name = da.attrs['standard_name']
    elif da.name is not None:
        name = da.name
    else:
        name = ''

    if da.attrs.get('units'):
        units = ' [{}]'.format(da.attrs['units'])
    else:
        units = ''

    return '\n'.join(textwrap.wrap(name + extra + units, 30))