Percent Change Between Values on Line Chart

hi  TimNYC 

I think the best way is add it into Tooltip of line chart.

and there is another workaround for you refer to:

Step1:

Use combo chart instead of line chart in the report.

https://docs.microsoft.com/en-us/power-bi/visuals/power-bi-visualization-combo-chart

Step2:

Create different measure by year, eg.

2019 = CALCULATE(SUM('Table'[Value]),FILTER('Table','Table'[Year]=2019))

2020 = CALCULATE(SUM('Table'[Value]),FILTER('Table','Table'[Year]=2020))

Step3:

Drag these measures into line values.

Step4:

Create a Percent Change measure like below:

Difference = 
var lastyear=  CALCULATE (
        SUM ( 'Table'[Value] ),
        FILTER ( ALL ( 'Table'[Year]), 'Table'[Year] = MAX ( 'Table'[Year] ) - 1 )
    )
return

DIVIDE(CALCULATE(SUM('Table'[Value]))-lastyear,lastyear,BLANK())

Then drag this measure into column values

Step5:

You could change the color of the column to white and custom the two Y-axis in format.

Result:

and here is sample pbix file, please try it.

Regards,

Lin