To read the graph:

Since you are checking under no load (NLD) conditions, the Y-axis on the graph collapses to the NLD 'isobar' line. Timing scale is indicated as 5 degrees/div, so draw lines parallel to the X-axis to find their intersection with the NLD line. This will give you - to the precision of this graph - the rpm that corresponds to each 5 degrees of advance.

Steve