When grid lines are displayed, they should be unobtrusive and not obscure data elements (e.g., curves and plotted points).
Additional Information:
Grid lines should be thinner than data curves, and should be invisible behind depicted objects and areas. Heavy grid lines may conceal details of plotted data. Electronic displays offer more flexibility than printed graphs. Grids can be displayed or suppressed by user selection. For reading the value of a particular data point, perhaps no grid is needed at all. A user might simply ask the computer to display the value of any selected point.