Help with pie chart formatting...?

Hi all, this may be a tad difficult to explain.

Essentially i want to have a pie chart where there is a 1/4 filled in when a person has achieved 25% towards a certain level of training.

So, if i was fully competent, my pie chart would be at 100% and hence fully coloured in. At 25%, it would be 25% coloured and the rest white or something along those lines.

From this i would like to be able to have a drop down which says 25/50/75/100 % and this would automatically bring up the pie chart mentioned above.

Is this possible?

Cheers as always

Simon