Problem with ages

I'm trying to creating charts showing reading ages of school pupils in several classes. They're in the format yy.mm, so 7.02 is 7 years 2 months, 13.11 is 13 years and 11 months etc. Is there a way to persuade Excel 2010 that the sequence goes 7.00, 7.01, 7.02 ... 7.10, 7.11, 8.00 without it leaving a gap from 7.12 to 7.99?

Sorry if this has already been answered: I did search but couldn't find it.

I've got several groups of pupils with a range of reading ages - say:
Group H1 has reading ages 12.07, 9.08, 11.10, 12.02, 14.05, 11.01, 13.02, 10.03, 9.04, 11.08, 15.06, 12.01, 11.05
Group H2 has reading ages 10.00, 9.01, 11.06, 9.08, 9.08, 10.06, 10.03, 9.05, 10.06, 8.08, 11.05, 13.08
Group F2 has reading ages 8.11, 7.01, 6.09, 8.10, 7.06, 10.11, 9.07, 7.05, 6.08, 9.03, 10.07

I want to create a scatter graph with each group's dots in a different colour, showing the outliers who don't belong in that group. But if I enter the data above and create a scatter graph, Excel doesn't produce the expected single cloud but instead a series of smaller cloud bands with empty space between them, caused by what it perceives as the huge gap between 9.11 and 10.

I want it to display the number as yy.mm, but not to expand that to a date like 2012.10.

Maybe what I need is the ability to calculate in duodecimal. Can Excel do that? I could manually convert 9.11 to 9B.