Linear trend line for accumulated values

[Go-Anna]

I have used this formula:

linest_m(total aggr(if(sum([Hours Worked]),sum([Hours Worked])),Period),Period)*only({1}Period)+linest_b(total aggr(if(sum([Hours Worked]),sum([Hours Worked])),Period),Period)

If I accumulate both my "worked hours" measure and my linear measure above, then the line is no longer linear? How can I make it straight again?

Thanks,