 We show that deciding whether there exists a realization of a given graph as unit disk graphs on straight lines is NP-hard, even if the given lines are parallel to either x-axis or y-axis.
<coherence>  We show that deciding whether there exists a realization of a given graph as unit disk graphs on straight lines is NP-hard, even if the given lines are parallel to either x-axis or y-axis.
 We show that deciding whether there exists a realization of a given graph as unit disk graphs on straight lines is NP-hard, when the given lines are parallel to either x-axis or y-axis.