'Return the state of the sensor.'

@property
def state(self):

    data = self._api.data['volumes'][self.monitor_device]
    free_gb = (((int(data['free_size']) / 1024) / 1024) / 1024)
    if (self.var_id == 'volume_size_free'):
        return round_nicely(free_gb)
    total_gb = (((int(data['total_size']) / 1024) / 1024) / 1024)
    used_gb = (total_gb - free_gb)
    if (self.var_id == 'volume_size_used'):
        return round_nicely(used_gb)
    if (self.var_id == 'volume_percentage_used'):
        return round(((used_gb / total_gb) * 100))
