gradient of regression - multiple dimensions

[#1]

Following on from my prior question, id like to extract the gradient of a line of best fit of a scatter graph.

i am using LINEST_M and have a gradient which seems to be correct except that my chart has multiple dimensions but im not sure how to declare these in the statement

=LINEST_B(aggr(avg({<parameter={'Y'}>}value),P1),aggr(avg({<parameter={'X'}>}value),P1))

here, P1 is a field, but i need to put P1, P2, P3 in there (in same way you put 3 in the dimension area of the scatter chart)

thanks in advance