Increase resolution ggplot charts

问题:

Using this code:

 ggplot(BP, aes(x=1, y=Enrichment_Score, fill=GO.ID)) +
    geom_bar(stat="identity") +
    coord_polar(theta='y') +
    guides(fill=guide_legend(override.aes=list(colour=NA), nrow=6)) +
    theme(axis.ticks=element_blank(), axis.title=element_blank(), axis.text=element_blank(), panel.grid  = element_blank(), legend.position="bottom") +
    ggtitle("Biological Processes")

I'd like to increase the resolution and, thus, the quality of this plot... How might I do this?