Plotting number and text onto the same chart

Hello,
o
I'm currently working on a chart to present some data using excel.

I work as an educator and want to present my students academic growth year on year, the basic way our levels system works is such:

1i, 1ii, 2i, 2ii, 3i, 3ii, 4,5,6,7,8, 1C, 1B, 1A, 2C, 2B, 2A


Now I'm happy to present 1i as 1.1 and this works well for my purposes, however when I have a student who has level 1C for example I can't plot it on the same chart, what I think I need is this - a way to display 1C in the table, but report the value as '9' to the chart so that it appears correctly in sequence.

Is anyone able to offer a clever answer to this? It's driving me mad, I mean at a pinch I can display 1C as 9 in the table but frankly it's unprofessional and bothers me.

Please help!