This is Harsh Dabav from Ahmedabad, India. I would like to share that there is bug in line chart as mentioned below.
Let us consider there are 3 lines in line chart. 1 line is mapped with Primary Y axis and other two lines mapped with secondary Y axis. We are able to set data colour for each lines as per our need. But, we are not able to set marker colour with same with line colour which those mapped in secondary Y axis, It is taking only first line colour. There is cutomize series option is available but it's is not working properly for secondary Y axis. Yes, defenitely we are able to with same exercise with Primary axis by customize series option.
So request to update and allow this feature in secondary axis of line chart in next release.
Please feel free to reach me in case of any query.
Email : firstname.lastname@example.org
JFYI, I am using May - 2020 version (which is latest version) of Power BI Desktop.