Calculate Monthly Usage using multiple filters in Power Query

Hi HopkiJ ,

Please follow the steps below:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("fY4xCoAwEAS/IqkFk03ucnmElaVYprAV/49ENDlE0u3CMMy6mmU/8+DMaOZ85mNwZYImSxMsynEOZhsfEBUsC1FxwmRDI5USX6VISl5ZfWXLAhQaowT+tb6hXEPhba+UVSlJt5R1aQi90obSXbpd", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Site Name" = _t, #"Meter Ref" = _t, Date = _t, Reading = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Site Name", type text}, {"Meter Ref", type text}, {"Date", type text}, {"Reading", Int64.Type}}),
    #"Changed Type with Locale" = Table.TransformColumnTypes(#"Changed Type", {{"Date", type date}}, "fr-DZ"),
    #"Sorted Rows" = Table.Sort(#"Changed Type with Locale",{{"Site Name", Order.Ascending}, {"Meter Ref", Order.Ascending}, {"Date", Order.Ascending}}),
    #"Grouped Rows" = Table.Group(#"Sorted Rows", {"Site Name", "Meter Ref"}, {{"Data", each Table.AddIndexColumn(_,"Index",1,1)}}),
    Custom1 = Table.TransformColumns(#"Grouped Rows",{"Data",(x)=>Table.AddColumn(x,"Monthly Usage",each try (x[Reading]{[Index]}-x[Reading]{[Index]-1}) otherwise null )}),
    #"Expanded Data" = Table.ExpandTableColumn(Custom1, "Data", {"Date", "Monthly Usage"}, {"Date", "Monthly Usage"})
in
    #"Expanded Data"

Best Regards,
Gao
Community Support Team

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Thank you Gao, the help is greatly appreciated.

I followed the above and it all looks correct except my "Monthly Usage" values have come out at null

Can you tell where I have gone wrong? 

= Table.TransformColumnTypes(#"Changed Type2", {{"Date", type date}}, "en-GB")
 

= Table.Sort(#"Changed Type with Locale",{{"Site Name", Order.Ascending}, {"Meter Ref", Order.Ascending}, {"Date", Order.Ascending}})

= Table.Group(#"Sorted Rows", {"Site Name", "Meter Ref"}, {{"Data", each Table.AddIndexColumn(_,"Index",1,1)}})
 

= Table.TransformColumns(#"Grouped Rows",{"Data",(x)=>Table.AddColumn(x,"Monthly Usage",each try (x[Reading]{[Index]}-x[Reading]{[Index]-1}) otherwise null )})
 

= Table.ExpandTableColumn(Custom1, "Data", {"Date", "Monthly Usage"}, {"Date", "Monthly Usage"})