dsstats plugin last data

[dolla]

Hi,

I use dsstats plugin for weathermap plugin. In my cacti 0.8.7g, PIA 2.9, dsstats 1.4, I found that if a host is down for a while, the weathermap still show the last available-to-read numbers for some network graphs.
for instance, the table shows:

mysql> select * from data_source_stats_hourly_last where local_data_id = 2162;
+---------------+--------------------+------------+------------+
| local_data_id | rrd_name | value | calculated |
+---------------+--------------------+------------+------------+
| 2162 | netBytesRecPerSec | 120 | 120 |
| 2162 | netBytesSentPerSec | 238 | 238 |
| 2162 | netBytTotPerSec | 358 | 358 |
| 2162 | netCurBandwidth | 1000000000 | 1000000000 |
| 2162 | netOutQueueLength | 0 | 0 |
+---------------+--------------------+------------+------------+
5 rows in set (0.00 sec)

but in the rra, they are all "nan" already.
Is there any setting to tweak this behavior in dsstats plugin?
Thanks.