Recently on twitter @MaxDuley asked how to display trend lines, and only show them, but not the underlying marks. My reply had to do with changing the transparency of the underlying marks. Unfortunately, I was a bit brief and obtuse about how to do this.
More specifically, take a look a the attached images for one example of how to do this. The basic idea is to use the Color shelf on the Marks panel to set the transparency to 0%. This will completely fade out the actual marks, but leave the trend lines. This works regardless of the number of trend lines. Also, try varying the transparency so that the marks fade to some desired level, thus giving the appearance that they are screened or in the background.
Note that in this example, Measure Names is on the Color Shelf and Measure Values is on the Row Shelf because I was using more than one measure field on the Y-axis. However this works if you have even only one measure on the Y-axis and any dimension on the Color Shelf. Also, if you have no dimension on the Color Shelf, this technique will work on one trend line.
Hope this helps.
Peace and All Good!