Dynamically adjusting Graph's Legend to compare past two years

Add a column onto your date table, you could add a calculated column in DAX like

Current or Previous Year =
'Date'[Year] = YEAR ( TODAY () ) || 'Date'[Year] = YEAR ( TODAY () ) - 1

or could do the same thing in Power Query or SQL.

Use that column as a filter on the chart visual.

Create a calculated column in your Date table for relative year label

Is Rolling 2 Years =
IF (
    'Date'[Year] >= YEAR ( TODAY () ) - 1
        && 'Date'[Year] <= YEAR ( TODAY () ),
    1,
    0
)

Then in the visual, add Is Rolling 2 Years to Filters on this visual and set it to is 1. Done. Updates automatically every year with no maintenance needed.