how to show percentage of individuals for two groups on histogram?

Hello,

I have a data frame like this:

head(a)

         FID   IID FLASER PLASER DIABDUR HBA1C ESRD   pheno
1 fam1000-03 G1000      1      1      38  10.2    1 control
2 fam1001-03 G1001      1      1      15   7.3    1 control
3 fam1003-03 G1003      1      2      17   7.0    1    case
4 fam1005-03 G1005      1      1      36   7.7    1 control
5 fam1009-03 G1009      1      1      23   7.6    1 control
6 fam1052-03 G1052      1      1      32   7.3    1 control

dim(a)

[1] 1698    8

I am doing histogram plot via:
ggplot(a, aes(x=HBA1C, fill=pheno)) + geom_histogram(binwidth=.5,
position="dodge")

there is 848 who have "case" in pheno column and 892 who have
"control" in pheno column.

I would like to have on y-axis shown percentage of individuals which
have either "case" or "control" in pheno instead of count.

Please advise,
Ana