I have no idea if this what you are looking for, but if I understand your request, this works for me:
In A1...Ax: the date of your flight
In B1...Bx: the time your flight ended (Blocks ON, or as per your needs)
In C1...Cx type: =A1 +B1 (format this cell as dd-mmm-yy or similar mmm-dd-yy)
In the cell you need the answer, type: =SUMIF(C1:Cx,”>=”TODAY()-28,C1:Cx)
This will give you the flight time (or whatever your requirement) over the last 28 days (or whatever time lapse you decide)
Regarding your second question (graph), I never looked at it. nevertheless, you could dedicate a page (tab) (hiden at a later stage, dedicated to some calculations replicating the above formulae (with adjusted time lapse) and laid out into a table from which your could extract a graph displayed on a visible tab.
A bit of work and may be not very aesthetic but it might reach your goal.
MP