You need points along the Y axis (X=14, the minimum, right? or 0.5 if X is a
category axis, but I hope from your description that it isn't). The points
occur at Y = 10, 16, 22, 28, 34, 40, 46, 52, 58, 64, 70, 76, 82, 88, 94,
100. Which of these need a letter? Suppose this covers it:
X   Y  Letter
14  40   E
14  52   D
14  64   C
14  76   B
14  88   A
14 100   A*
Use the X and Y columns to add a series (copy, select chart, Edit menu >
Paste Special, New series), convert the series to an XY if necessary (select
the series, Chart menu > Chart Type > XY markers only). Change the markers
to cross (+), use one of these utilities to apply labels from the Letter
column to the added series:
Rob Bovey's Chart Labeler, 
http://appspro.com
John Walkenbach's Chart Tools, 
http://j-walk.com
Hide the regular axis labels (double click > Patterns tab > Axis Tick Labels
= None).
- Jon