Problem with TDBChart(lines)

Hi,

(TDBChart in Delphi 3 Prof.  )

Problem: I am representing a sequence of values by a line, taking them of a
TTable. But, if in a tract the value doesn't exist (the TTableField is
null), the line should not be drawn.  TDBChart always draws the lines of
the tracts, taking the value zero for the fields (numeric) with null value.

I need thar when a tract doesn't have value, the graph of the line it
should not be
drawn, because in this tract it doesn't have the value zero, but rather it
is not defined.  

Any idea? Thanks in advance.