You can duplicate the measure you are using on the chart and create a new one to show data from 2020 to 2023 as adotted line. The original measure reflects data from 2020 to 2023 and is solid color. You add both measures and create a dual axis charts. Then under marks you can edit the line before the 2023 data and after. Otherwise you can't make a semi solid, semi dotted (yet).