Refresh Data - Append New Data Only

A workaround - maybe strange, questionable and controversial, but still a wordkaround - would be to fix your data by copying and pasting into "PreviousData" and then modify your query to have new data appended to the previous data.

After each refresh, you need to copy and paste into PreviousData.

I guess it would be best to use that dataset as the source for your reports (rather than the new dataset), so you won't forget to copy the data to PreviousData.

Edit: it is assumed that "Date" is the primary key. Adjust as appropriate, 

let
    Source = Csv.Document(File.Contents("C:\Users\Marcel\Documents\Forum bijdragen\Power BI Community\Append new Data only\Inputdata.csv"),[Delimiter=",", Columns=2, Encoding=1252, QuoteStyle=QuoteStyle.None]),
    #"Promoted Headers" = Table.PromoteHeaders(Source),
    NewData = Table.TransformColumnTypes(#"Promoted Headers",{{"Date", type date}, {"Value", Int64.Type}}),
    Merged = Table.NestedJoin(PreviousData,{"Date"},NewData,{"Date"},"NewColumn",JoinKind.RightAnti),
    Combined = if Table.IsEmpty(Merged) then PreviousData else Table.Combine({PreviousData, Merged[NewColumn]{0}})
in
    Combined

Steps are illustrated in this video:

Sorry, This feature not yet implemented in Power BI.

I think they are working on it. 

If u r facing this prob i personally suggest u go with SSAS Tabular cube Live connection that will help u