formula to enable plotting aggregated data

You're not a million miles off in truth...

You could use SUM(INDEX:INDEX), eg:

B2: =SUM(INDEX(A:A,2+(4*(ROWS($B$2:$B2)-1))):INDEX(A:A,2+(4*(ROWS($B$2:$B2)))-1))

Or

B2: =SUM(OFFSET($A$1,1+(4*((ROWS($B$2:$B2)-1))),0,4,1))

The advantage of the former over the latter is that the former is not Volatile whereas the latter is (OFFSET).

Given we don't know what it is you're doing it's hard to offer anything more but it might be feasible to use a Pivot Chart and Group the dates ?

Thanks for that, I'll go with the OFFSET version.
I would never have worked that out in a million years.

Was the first one strictly correct ?

Dave

Was the first one strictly correct ?

Not sure I follow...the first function (INDEX:INDEX) should work ok, no ?

INDEX:INDEX should really be used in preference to OFFSET given the latter's volatility - if your model is not big it's not a huge issue but it's generally advised that you should avoid using Volatile functions wherever/whenever possible... I accept OFFSET may be shorter/easier to follow but not necessarily the best approach in this instance.

The simplest way would be to test... if you look very closely you will find they're not the same (for ex. count the parentheses prior to the -1) ...