I also added 'None Selected' to the top of the list and forced a carriage-return after it, by including a comma. Since the field that I am using actually has commas in the field values themselves, I had to replace the comma with an underscore ("_"), then swap them back at the time of use.
I implemented the replacement with the underscore (which is not ideal because it results in showing the underscores in the drop-down menue) and am faced with the re-replacement at time of use. I tried the following, but I can see that the editor does not recognise the replacement()-function as a function (it's not turned blue). I wondered what is wrong with the syntax and how I can correct it. (FYI, I use this expression in the Background colour expression for a grid chart to set the colour of the bubbles.)
Note: Highlight is the Variable set using the drop-down menue in the Input Box, Var2 is the field that Highlight is compared against to limit the selection. Var1 is the Dimension that determines the x-axis in the grid chart.
Just correcting the code would be fine for me at this stage. Having a more elegant solution that allows to actually show comma values in the Input Box drop-down menue would be even better.