Labeling collinear sites

We consider a map labeling problem, where the sites to be labeled are restricted on a line L. This is quite common e.g. in schematized maps for road or subway networks. Each site si is associated with an axis-parallel wi hi label li, which can be placed anywhere on the boundary of the input line L. The main task is to place the labels in distinct positions, so that they do not overlap and do not obscure the site set, and to connect each label with its associated site through a leader, such that no two leaders intersect. We propose several variations of this problem and we investigate their computational complexity under certain optimization criteria