First, pie charts is not the best way to show your data!
But anyway here is one solution. Most important part here is scale_y_continuous()
to set limits from 0 to 1 and then convert them to percents using percent_format()
from library scales
.
library(ggplot2)
library(scales)
ggplot(df,aes(x=1,y=Errors/Fields))+geom_bar(stat="identity")+
facet_grid(~Hospital)+
coord_polar(theta="y")+
scale_y_continuous(labels = percent_format(),limits=c(0,1),
breaks=c(0.25,0.5,0.75,1))+
theme(axis.title.y=element_blank(),
axis.text.y=element_blank(),
axis.ticks.y=element_blank())