sum of the graphs

[guest]

HI,

When i try sum two graphs,i get the following results:

RRDTool Says:

Notice: Undefined offset: 2 in C:\Apache2\htdocs\cacti\lib\rrd.php on line 381

Notice: Undefined offset: 2 in C:\Apache2\htdocs\cacti\lib\rrd.php on line 385

Notice: Undefined offset: 2 in C:\Apache2\htdocs\cacti\lib\rrd.php on line 389
OK

CAn someone help me?
How i can sumtwo graphs?
What steps I have to make?

Thans you ¡