I guess given the name of the plugin this makes sense since most of the time you are filling an area. However, there are times when we want to distribute items along a line. It would be great if there was a way to specify the way thing are to be distributed like area or line and distance, number or volume. So for density first select either area. If you select area it would work like the current method. If you select line (or some other term) you would have options for specifying the number of objects or the distance between the objects and they would use the length of the “line” to place the objects. If you look at my thread Place objects in a straight line I think you will see the issues I am trying to point out. 