与图例本身相比，有没有办法将图例标题放在顶部和中心？
例子：
cons2 <- data.table(
value_date = c(as.Date('2013-04-30'),as.Date('2013-04-30'),as.Date('2013-06-13'),as.Date('2013-06-13')),
ticker = c('AAPL','FTW','AAPL','FTW'),
discount = c(0.34,0.10,0.25,0.20),
b = c(0.40,0.55,.60,0.90),
yield = c(0.08,0.04, 0.06,0.03))
headers <- cons2[ticker == 'AAPL' & value_date == '2013-06-13']
p <- ggplot(cons2)
p <- p + geom_point(aes(yield,b, size = discount, color=factor(value_date)))
p <- p + guides(size = "none")
p <- p + scale_colour_manual(values = c("#1c2f80","#779438"))
p <- p + geom_text(data = headers, aes(yield, b, label=ticker), size = 4)
p <- p + geom_smooth(data = cons2[value_date == '2013-06-13'], aes(yield, b), method="lm", se=FALSE)
p <- p + geom_line(data = cons2, aes(yield, b, group = ticker))
p <- p + theme(legend.position="bottom")
p <- p + guides(colour=guide_legend(title.position="top"))
p
