How to draw a reference line with transparent filling?

[redsun]

when I am trying to add a reference line (y=constant) in my polar polt using "RefLine" command, I find that the reference line(a circle) is always filled with blue corlor. Would you please tell me which command can remove the corlor filling and the reference circle becomes tansparent. thanks.

[DPlotAdmin]

That's certainly not by design and isn't what I get when trying it just now. Please save your graph as a DPlot file and send it to me.