I don't believe you'll be able to do that with the current data structure. If you were to pivot the data so there was a column for Letter and another for amount, then it would be easy. It would also be possible if you only had two items to compare as you could do a dual axis (a dual axis would allow you to customize the tooltip for each metric). With this many members it requires you to use Measure Names and Measure Values, which doesn't allow for custom tooltips on each line, and would only be possible with a dimension (A, B, C) to direct it.
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Thank you Deepak!