删除图例项目周围的灰色框

Question

我尝试绘制以下数据：

MONTH;MEDIAN;MIN;MAX
1;-736.12;-5272.96;5946.79
2;3340.83;284.72;15707.46
3;7144.85;2916.79;17659.6
4;9927.73;1660.99;19191.29
5;10986.7;3689.15;16474.34
6;1526.48;-8086.48;13430.86
7;-4415.36;-9244.11;27554.34
8;-3213.32;-8970.76;22089.98
9;-3435.17;-5293.95;12451.59
10;-5112.405;-7002.67;5237.85
11;-7820.8;-11170.12;1164
12;-6143.055;-10836.93;165.82

使用以下代码：

png("amazon_ghm.png", width = 600, height = 400)
ggplot(amazon_ghm) +
  geom_line(aes(MONTH, MEDIAN, colour = 'Median'), group=1, size = 2) +
  geom_ribbon(aes(MONTH, ymax = MAX, ymin = MIN, fill = "Min/Max"), alpha = 0.5) +
  geom_hline(aes(yintercept = 0), linetype="dotted") +
  geom_text(size=8, aes(6, 28000, label = "U. Amazon, GHM"))+
  theme_bw() +
  theme(axis.text=element_text(size=20),
        axis.title=element_text(size=20),
        axis.line = element_line(colour = "black"),
        legend.key = element_rect(fill = "white"),
        legend.position = c(.9, .8)) +
  labs(x = "Month",
       y = "Diff in runoff [m3/s]")+
  scale_x_continuous("Month", breaks = 0:12, expand = c(0,0.5)) +
  scale_y_continuous(limits = c(-15000, 30000)) +
  scale_colour_manual(values = c('Median' ='red4'), name = '')+
  scale_fill_manual(values = c('Min/Max' = 'tomato1'), name = '')

不幸的是，当我看到图例项目时，周围会有一些灰色的盒子。我尝试了legend.key = element_rect(fill = "white")命令，但没有成功。

有谁知道如何摆脱图例项目周围的灰色框？