Last 12 Months Column in Power Query

Here's one way to do it in the query editor.  To see how it works, just create a blank query, open the Advanced Editor and replace the text there with the M code below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Xc9LDsAgCATQu7A2cUBt7LK9hvH+1xA/qZbFhMVLBiiFcHuwFwiTo0fDI9UVYjEkmjAIYZEsSpo8KRnCpnjQO8q+wmyo74zzDBjCJjbU5/U/IxyF+ldt", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Reference Date" = _t, Component = _t, #"Monthly Removals" = _t, #"Removals L12M" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Reference Date", type date}, {"Component", type text}, {"Monthly Removals", Int64.Type}, {"Removals L12M", Int64.Type}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each let
thisdate = [Reference Date],
thiscomponent = [Component],
prev12mrows = Table.SelectRows(#"Changed Type", each ([Component] = thiscomponent and ([Reference Date] <= thisdate and [Reference Date] >= Date.AddMonths(thisdate, -12)))),
sum = List.Sum(prev12mrows[Monthly Removals])
in 
sum)
in
    #"Added Custom"

Pat