ggplot2 stat_function图例标签的顺序

Question

在涉及指定因子级别时更改ggplot2的图例标签的顺序，如this example中所示：

diamonds$cut <- factor(diamonds$cut, levels = rev(levels(diamonds$cut)))

我正在尝试更改使用以下代码生成的此绘图中stat_function图层的图例标签的顺序：

myfun1 <- function(x) -.167*x + 0.032*x^2 -0.002*x^3 + .694 # low
myfun2 <- function(x) -.167*x + 0.032*x^2 -0.002*x^3 + .694 + 0.249 # mid vs. low
myfun3 <- function(x) -.167*x + 0.032*x^2 -0.002*x^3 + .694 + 0.492 # high vs. low

df <- data.frame(x = c(0, 8), y = c(0, 8)) # make highest first

myplot <- ggplot(data = df, aes(x = x, y = y)) + # maybe need to change x-axis labels
  stat_function(fun = myfun1, aes(color = "Lowest")) +
  stat_function(fun = myfun2, aes(color = "Average")) +
  stat_function(fun = myfun3, aes(color = "Highest")) +
  scale_x_continuous(expand = c(0, 0), limits = c(0, 8)) + 
  scale_y_continuous(expand = c(0, 0), limits = c(0, 1.5)) +
  theme_minimal() +
  ylab("Proportion of Videos Watched") +
  xlab("") +
  scale_colour_brewer("Students' Course Grade", type = "qual", palette = 4, direction = -1) +
  theme(legend.position = "right", 
        legend.direction = "vertical") +
  theme(text=element_text(size = 16, family= "Times")) +
  scale_x_continuous(breaks = 0:8,
                     labels = paste0("Wave ", 1:9)) +
  theme(axis.text.x = element_text(angle = 45, hjust = 1))

myplot

如何更改这些stat_function图例标签的顺序，以便对它们进行排序Lowest，Average，Highest？