TOP N affected by Legend in Bar Graph

When you add Participant to the Legend, the visual evaluates your measure for every Dealer + Participant combination. If you do not want it to affect the visual you could try using REMOVEFILTERs() inside your measure:

Top N Spend =
VAR _TOPNSelected =
    SELECTEDVALUE ( 'Top N'[TopN] )
VAR _CurrentDealer =
    SELECTEDVALUE ( 'Counterparty List 2'[Dealer] )
VAR _TopDealerTable =
    TOPN (
        _TOPNSelected,
        ALLSELECTED ( 'Counterparty List 2' ),
        CALCULATE (
            [Metrics],
            REMOVEFILTERS ( 'Dealer Ranking'[Participant] )
        )
    )
VAR _IsTopDealer =
    COUNTROWS (
        INTERSECT (
            VALUES ( 'Counterparty List 2'[Dealer] ),
            SELECTCOLUMNS ( _TopDealerTable, "Dealer", [Dealer] )
        )
    ) > 0
VAR _TopDealerCommission =
    CALCULATE (
        [Metrics],
        KEEPFILTERS ( _TopDealerTable )
    )
VAR _OtherCommission =
    CALCULATE (
        [Metrics],
        ALLSELECTED ( 'Counterparty List 2' )
    ) -
    CALCULATE (
        [Metrics],
        _TopDealerTable
    )
RETURN
    IF (
        _CurrentDealer <> "Others",
        IF ( _IsTopDealer, _TopDealerCommission ),
        _OtherCommission
    )