If interpolation must be made or where accuracy of reading graphic data is required, computer aids should be provided for exact interpolation.
Additional Information:
It might suffice, for example, to allow users to request a fine grid as an optional display feature. It might be better to display vertical and horizontal rules that a user could move to intersect the axes of a chart. It might prove best simply to let a user point at any data item and have the computer label that item with a readout of its exact value(s).