For line chart, we can't add conditional formatting on it. You can do that on clustered column chart. However, you can only add one measure in the y-axis field. You can create a measure

Measure = sum('Table'[actual])-sum('Table'[plan])

then set up the conditional formatting

at last, you can the chart to line chart

As you can see, we can only set up the color for marker. We can't do it for line.