values that appear in a bar chart Legend

[cseward1963]

is it possible to restrict values in the legend only to those that appear in the bar chart?   Not all grades are represented in the chart, but all grades appear in the legend.  See below. 

ALSO, i'm wondering if there is some way I can assign a "sort value" (I just made up sort value) to each grade so that Pre-K and K appear before grade 1 and 12+ appear after grade 12.   I'm thinking something like Pre-K=-1, K=0 and 12+ = 13 - do you see what I mean.