Draw graph with more curves

Hi All,
I'm not expert to Crystal Reports. I'm using Crystal Reports 2008.
I need to show two graphs, each graph with different pens (curves).
Data are in two tables vTags and vTagsValues.
vTags:
Path String
Tag_ID Number

vTagsValues:
Tag_ID Number
TagValue Number
TimeStamp Date/time

Path field identifies the group: FERMENTAZIONE (contains string "FERMENTAZIONE$")
and MATURAZIONE (contains string "MATURAZIONE$").
So each Tag_ID belongs to one of two groups.
For each Tag_ID I need to draw one curve that shows the couple
TimeStamp / TagValue.
TimeSpamp may be different for each Tag_ID.
I used crosstab table with Tag_ID as column, TimeStamp as row and
the sum of TagValue as crosstab value.
Then I used the crosstab value to draw the graph.
The problem is that the table contains the value 0
for the Tag_ID who have not sampling for that
TimeStamp (the sum is 0).

For each Tag_ID, how can I remove the 0 values and draw
the correct couple TimeStamp / TagValue in the graph?

Help me please...
Thanks in advance