Dynamically Highlight Selected City in Line Graph Using DAX for Conditional Formatting

Hi manialla 

What you're trying to do requires conditional formatting on legends which currently isn't supported. There is a workaround which may work for a few number of items or more (if you have the patience to manually set the legend color). This requires using two disconnected tables to return the category value/legends.

Below is a sample measure that references two disconnected tables.

Value2 = 
VAR _value =
    CALCULATE (
        [Sum of Value],
        TREATAS ( VALUES ( City02[City] ), 'Table'[City] ),
        TREATAS ( VALUES ( City01[Area] ), 'Table'[Area] )
    )
VAR _value2 =
    CALCULATE ( [Sum of Value], TREATAS ( VALUES ( City02[City] ), 'Table'[City] ) )
VAR _areaCheck =
    ISFILTERED ( City01[Area] )
VAR _selected =
    IF (
        SELECTEDVALUE ( City02[Category] ) = "selected",
        IF ( NOT _areaCheck, _value2, _value )
    )
VAR _unselected =
    IF (
        SELECTEDVALUE ( City02[Category] ) = "unselected",
        IF ( NOT _areaCheck, _value2, _value )
    )
RETURN
    IF (
        NOT ( HASONEVALUE ( City01[City] ) ) || NOT ( ISFILTERED ( City01[City] ) ),
        _unselected,
        IF (
            SELECTEDVALUE ( City02[City] ) IN VALUES ( City01[City] ),
            _selected,
            _unselected
        )
    )

 Please see the attached sample pbix.