All the graphs are display correctly except one graph

Hi,
    The problem is that all the graphs are display correctly except one
graph.
    Graph A and Graph B display correctly.
    Graph C contains all the items in Graph A and Graph B, but Graph C
cannot display. I've checked all the 329 items in Graph C. It seems nothing
wrong here. Where could be the problem?

    Another question. This Cacti have run for several years, but the
cacti.log is blank. How to set the cacti to write log info into cacti.log?

    System infomation:

Cacti Version0.8.7bPlugin Architecture2.1Poller TypeCactid vServer InfoLinux
2.6.18-348.12.1.el5Web ServerApache/2.2.3 (CentOS)PHP Version5.1.6PHP
Extensionslibxml, xml, wddx, tokenizer, sysvshm, sysvsem, sysvmsg,
standard, SimpleXML, sockets, SPL, shmop, session, Reflection, pspell,
posix, mime_magic, iconv, hash, gmp, gettext, ftp, exif, date, curl, ctype,
calendar, bz2, zlib, pcre, openssl, apache2handler, dbase, gd, ldap, mysql,
mysqli, PDO, pdo_mysql, pdo_sqlite, snmpMySQL Version5.0.95RRDTool Version
1.3.4SNMP Version5.3.2.2

-- 
Cheers,

Pei Fei
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