As you know, Photshop is a pixel based system, a series of boxes (or picture cells, pixels), as seen in the enlarged image fragment above, each box so small that, as a cluster, they seem to fuse, rather than being seen as discrete boxes. Each cell cannot be bisected but rather is assigned a color value in its entirety. Since you are looking for something smaller than your assigned pixel size (resolution) I would suggest you move out of Photoshop for your graph and into Illustrator, a vector system, where measurements may be made in other units, as shown below.
