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To make marks more visible against a background image or map, surround each mark with a solid contrasting color called a halo. Mark halos are available when you have a background image or a background map. On the Color drop-down control, select a mark halo color.
In Tableau Desktop, when you are using the Line mark type, you can add a marker effect to show or hide points along the line. You can show selected points, all points, or no points. On the Color drop-down control, select a marker in the Effects section.
Note: This option is not currently available in Tableau Server or Tableau Online.
hope this will help.
I've got one workaround using reference line, hope this will server your purpose
Change the marks of your label from text to details.
right click on your secondary axis and add reference line, set the values as per the screen shot
Now little formatting required, right click on any mark and select format
Now set the color of shading as per your requirement
workbook attached for your reference. Let us know if this works.
Sample Workbook_v10.2.twbx 17.7 KB
This approach solved my issue. Many thanks to you
You're welcome glad I could help!