What I think(haven't tested) could be a working class for freenas/gui/reporting/rrd.py

rrd.py

class DiskTempPlugin(RRDBase):

    vertical_label = "Celcius"

    def get_title(self):
        title = self.identifier.replace("disk-", "")
        return 'Disk Temperature (%s)' % title

    def get_identifiers(self):
        ids = []
        for entry in glob.glob('%s/disk-*' % self._base_path):
            ident = entry.split('-', 1)[-1]
            if not os.path.exists('/dev/%s' % ident):
                continue
            if ident.startswith('pass'):
                continue
            if os.path.exists(os.path.join(entry, 'temperature.rrd')):
                ids.append(ident)

        ids.sort(key=RRDBase._sort_identifiers)
        return ids

    def graph(self):

        path = os.path.join(self._base_path, "disk-%s" % self.identifier, "temperature.rrd")

        args = [
            'DEF:min_tmp=%s:value:MIN' % path,
            'DEF:avg_tmp=%s:value:AVERAGE' % path,
            'DEF:max_tmp=%s:value:MAX' % path,
            'AREA:avg_tmp#bfbfff',
            'LINE1:avg_tmp#0000ff:Temp ',
            'GPRINT:min_tmp:MIN:%5.1lf%s Min\g',
            'GPRINT:avg_tmp:AVERAGE: %5.1lf%s Avg\g',
            'GPRINT:max_tmp:MAX: %5.1lf%s Max\g',
            'GPRINT:avg_tmp:LAST: %5.1lf%s Last\g',
        ]

        return args