Enable Line Segments from Multiple Directions in create.scatterplot Function

[maotian06]

The current create.scatterplot function has the option to add line segments using the following option:

add.line.segments = FALSE,  line.start = NULL, line.end = NULL

However, this function only allows adding the vertical line from the same location of x, but not the horizontal lines; here is the source code:

    if (add.line.segments) {
      for (i in 1:length(line.start)) {
        with(data = new.env(), expr = panel.segments(x, 
                                                     line.start[[i]][subscripts], x, line.end[[i]][subscripts], 
                                                     col = line.col[[i]], lwd = line.lwd[[i]], lineend = 1))
      }

I think the current version doesn't allow for the customization of x effectively. It would be useful to enhance the functionality by allowing the addition of multiple arguments, enabling features like drawing horizontal lines and lines from different positions, such as:

line.start.x = list(), 
line.start.y = list(), 
line.end.x = list(), 
line.end.y = list()

One potential application for this enhancement is in drawing enriched gene locations in the GSEA plot:

[WuSelina]

Yes, this! I was also spending a lot of time trying to verify if line segments can only be drawn vertically.

[pboutros]

This is a great example of a feature that we probably shouldn't add. There is another way to add arbitrary lines to a plot, which is by adding a panel.abline() directly to the final trellis object. For example, you could use the layer() function in latticeExtra. We don't want the panel function of create.scatterplot() or any equivalent to continually expand, but rather want people to use it for most cases, and then add on top of it using once of the standard trellis or lattice functionalities.