减小图例的大小并将图例项目放在两列中

Question

我有这个代码来绘制一些图表

library(grid);
library(ggplot2);
library(reshape2);
pdf(file = '$filename.pdf', width=5, height=6.7);

dat <- read.csv('bdr.csv');

ggplot(dat, aes(x = Experiment, y = Mean, colour = Method:factor(Values),
             linetype = Method:factor(Values), shape = Method:factor(Values))) +
    geom_line() +
    geom_point()+
    labs(x='$xlabel',y='$ylabel', fill='') +
    scale_shape_manual(name = 'Method and\nBeacon Send Rate', 
                labels = c('DTB-MAC, Beacon Send Rate:1HZ', 'DTB_MAC, Beacon Send Rate:5HZ', 'DTB_MAC, Beacon Send Rate:10HZ', 'IEEE802.11P, Beacon Send Rate:1HZ', 'IEEE802.11P, Beacon Send Rate:5HZ', 'IEEE802.11P, Beacon Send Rate:10HZ'), 
                values = rep(c(15,16,17), 2)) +
    scale_linetype_manual(name='Method and\nBeacon Send Rate', 
                  labels = c('DTB-MAC, Beacon Send Rate:1HZ', 'DTB_MAC, Beacon Send Rate:5HZ', 'DTB_MAC, Beacon Send Rate:10HZ', 'IEEE802.11P, Beacon Send Rate:1HZ', 'IEEE802.11P, Beacon Send Rate:5HZ', 'IEEE802.11P, Beacon Send Rate:10HZ'), 
                  values = rep(c('solid', 'dashed', 'dotted'), 2)) +
    scale_color_manual(name='Method and\nBeacon Send Rate',
                labels = c('DTB-MAC, Beacon Send Rate:1HZ', 'DTB_MAC, Beacon Send Rate:5HZ', 'DTB_MAC, Beacon Send Rate:10HZ', 'IEEE802.11P, Beacon Send Rate:1HZ', 'IEEE802.11P, Beacon Send Rate:5HZ', 'IEEE802.11P, Beacon Send Rate:10HZ'), 
                values = rep(c('red', 'deepskyblue'), each = 3)) +
    scale_y_continuous(limits = c(0, 100), breaks = (seq(0,100,by = 20))) +
    scale_x_continuous(breaks = c(10, 20, 40, 60, 80)) +
    theme_bw() +
    theme(panel.grid.major = element_line(colour = 'grey'),
     panel.border = element_rect(colour = 'black'),
     axis.line = element_blank(),
     panel.background = element_blank(), 
     legend.direction='vertical', 
     legend.key = element_blank(),
     legend.position='bottom', 
     legend.background = element_rect(colour = 'black'),
     legend.title=element_blank());

这是结果

但是如何减少图例的大小并将项目分成两列以避免变大？