Delta Calculation per Time Period for Group Values

[tester]

I've seen a hundred posts on this but no real solution. 

My data:

Group	Date	Value
a	1/13/2015	897
a	1/9/2015	200
a	1/1/2015	647
b	1/13/2015	2
b	1/9/2015	978
b	1/1/2015	655
c	1/13/2015	61
c	1/9/2015	36
c	1/1/2015	959

What I need: a delta calculation that subtracts the difference of each value from last week, for each group. Ergo:

Group	Date	Value	Delta
a	1/13/2015	897	697
a	1/9/2015	200	-447
a	1/1/2015	647	0
b	1/13/2015	2	-976
b	1/9/2015	978	323
b	1/1/2015	655	0
c	1/13/2015	61	25
c	1/9/2015	36	-923
c	1/1/2015	959	0

Note where the delta is 0.

This is easily achieved in R:

dataset %>%
group_by(Group) %>%
mutate(Delta = Value - lag(Value, order_by=Date))

Unfortunately there's no real enterprise support for R in PBI Service. So how do we do this with M/DAX?

[tester · Accepted solution]

Solution is here:

https://community.powerbi.com/t5/Desktop/Power-Query-previous-row-value-based-on-criteria-O/m-p/1258...