Expression from fields

[AmCh]

Hi all,

I have these fields as an example:

Field_a

1

2

3

Field_b_1

4

5

6

Field_b_2

5

6

7

Field_b

Field_b_1

Field_b_2

I want to create Field_c with the following expressions:

Field_c

=Field_a*Field_b_1

=Field_a*Field_b_2

Later I need in chart expression the value of Field_c (pre-defined expression). (by selecting Field_b_1 in Field_b, the related expression in Field_c should be computed).

How can I generate Field_c and how the expression in Field_c can be recignized in chart expression (by selecting Field_b_1 or Field_b_2 from Field_b) ?

Any help please?

Thanks in advance.