Abstract: Data processing apparatus for processing data relating to information represented by lines on a chart includes two stores, a computer, and a logic unit. One of the stores contains the cartesian coordinates of a plurality of spaced points along the lines while the other store contains the individual elements forming the lines between the spaced points. The computer operates to select sets of elements which may approach within a predetermined area around a point of interest on the chart, while the logic unit checks whether any element actually does fall within this predetermined area.