Possible Duplicate:
How to insert guide-lines in graphics without specifying range?
When we're using ArrayPlot, it's possible to draw a grid by doing:
ArrayPlot[{{1, 0, 0, 0.3}, {1, 1, 0, 0.3}, {1, 0, 1, 0.7}}, Mesh -> True]
But I can't find a way of doing it on Plot, ListPlot and ListLinePlot, can you help me?

