Diagonal reference line in histogram as a bar chart ?

[#1]

Dear all,

I would like to add a diagonal line (for reference) to a histogram (based on a bar chart).

What I managed so far is to create a line with the following formula (with date on the x-axis and sales on the y axis)

LINEST_M(total aggr(if(sum(SALES),sum(SALES)),date),date)*date+linest_b(total aggr(if(sum(SALES),sum(SALES)),date),date)

however, what I'd like to do is to compare predictions (simple movig average in this case: ewma) vs the average per prediction sales, and the following seems to work as well:

LINEST_M(total aggr(if(avg(SALES),avg(SALES)),ewma),ewma)*ewma+linest_b(total aggr(if(avg(SALES),avg(SALES)),ewma),ewma)

(ie replace date with ewma and sum with avg)

However, this produces a long big graph.. What I'd now like to do is to have a histogram, i.e. a binned x - axis like =Round(ewma,1).

I can do the histogram of the quantities alright but not the diagonal line....

Any help would be very welcome,

many thanks

Ulrich