Yes, it is possible to do this. It is controlled from the mask of the Show_Target block. If you specify the fill column as zero and provide a stroke width column then the fill is considered transparent. ex, set the Target 1 state value to something like [0 5 4 3]. That instructs the Show_Target block to use no fill, use column 5 of the target table for the stroke color, column 4 for the stroke width, and column 3 for the radius.