Viewing graph data values

I have a scatter graph with lots (thousands) of data points, and I am displaying it as a line with no data point markers.

when I click on the graph, a box appears near where I click, (see attached). Using the left and right cursor control keys I can move the box along the graph line.

I would like to know the value of the data point at the exact location of the box. Hovering the cursor gives the value at the cursor location, not the box location, and I would like to see how the values change as the box moves along the graph line. I think it would be neat if the corresponding cells in the worksheet would be highlighted as the box moves. Also, does anyone know what this box is called?