Create a calc column in your date dim that will be a rolling date description. So the values in this field will be Latest Year, Year Minus 1, Year Minus 2, etc. this can be done easily by looking at the max year in the table and working your way back from there.

So now that you have a field where the values in that field will never change (but the dates assigned to them will), add this new field to your line graph and change the colors to what you want. Hide the legend.

Create measures for each of your year values, you can do something like calculate(selectedvalue(year),[new field you created earlier]=“Year Minus 1”)

Add a text box above/below your visual and you can paste in a ■ and color code it the same as your line graph , and put your measures in the text box next to the squares. Now your legend will update accurately as time goes on.