ggplot2 - How to replace geom symbol in legend?

Here's a couple of approaches with ggnewscale::new_scale_color:

library(ggnewscale)
ggplot() +
  geom_ribbon(df2, mapping=aes(x=as.Date(jday, origin=as.Date("2020-01-01")), ymin=0, ymax=csum, fill=type), alpha=0.5) +
  geom_vline(aes(xintercept=as.Date(274, origin=as.Date("2020-01-01")), colour="Planting"), linetype="longdash") +
  geom_vline(aes(xintercept=as.Date(309, origin=as.Date("2020-01-01")), colour="Flowering"), linetype="longdash") +
  theme_bw(base_size=18) +
  scale_x_date(date_breaks="15 days", date_labels="%d/%b/%y") +
  labs(y="Cumulative rainfall (mm)", x= "Date") +
  theme(axis.text.x=element_text(angle=45, hjust=1)) +
  scale_color_manual(name="Phenological stages",
                     breaks=c("Planting","Flowering"),
                     values = c("Planting" = "#24CF09",
                                "Flowering" = "#805920"),
                     guide=guide_legend(nrow=1, byrow=FALSE,
                            override.aes = list(fill = "black"),
                            title.position="top", title.hjust=0.5)) +
  new_scale_color() +
  geom_line(df1, mapping=aes(x=as.Date(jday, origin=as.Date("2020-01-01")), y=csum, colour="current")) +
  scale_color_manual(name = "Rainfall", 
                     values=c("current"="steelblue3", "historical" = "skyblue1"),
                     labels = c("current"="Current year","historical"="Historical"),
                     drop = FALSE,
                     guide=guide_legend(nrow=1, byrow=FALSE,
                                        override.aes = list(fill = "black"),
                                        title.position="top", title.hjust=1.4)) +
  scale_fill_manual(name = "", values=c("current"="steelblue3", "historical" = "skyblue1"),
                    labels = c("current"="Current year","historical"="Historical"),
                    drop = FALSE,
                    guide = guide_legend(nrow=1, byrow=FALSE,
                                         title.position="top", title.hjust=0.5)) +
  theme(legend.position="bottom",
        legend.title=element_text(size=18),
        legend.text = element_text(size=18))

ggplot() +
  geom_ribbon(df2, mapping=aes(x=as.Date(jday, origin=as.Date("2020-01-01")), ymin=0, ymax=csum, fill=type), alpha=0.5) +
  geom_vline(aes(xintercept=as.Date(274, origin=as.Date("2020-01-01")), colour="Planting"), linetype="longdash") +
  geom_vline(aes(xintercept=as.Date(309, origin=as.Date("2020-01-01")), colour="Flowering"), linetype="longdash") +
  theme_bw(base_size=18) +
  scale_x_date(date_breaks="15 days", date_labels="%d/%b/%y") +
  labs(y="Cumulative rainfall (mm)", x= "Date") +
  theme(axis.text.x=element_text(angle=45, hjust=1)) +
  scale_color_manual(name="Phenological stages",
                     breaks=c("Planting","Flowering"),
                     values = c("Planting" = "#24CF09",
                                "Flowering" = "#805920"),
                     guide=guide_legend(nrow=1, byrow=FALSE,
                            override.aes = list(fill = "black"),
                            title.position="top", title.hjust=0.5)) +
  new_scale_color() +
  geom_line(df1, mapping=aes(x=as.Date(jday, origin=as.Date("2020-01-01")), y=csum, colour="current")) +
  scale_color_manual(name = "Rainfall", 
                     values=c("current"="steelblue3", "historical" = "skyblue1"),
                     labels = c("current"="Current year","historical"="Historical"),
                     drop = FALSE,
                     guide=guide_legend(nrow=1, byrow=FALSE,
                                        override.aes = list(fill = "black"),
                                        title.position="bottom", title.hjust=0.4)) +
  scale_fill_manual(name = "Rainfall", values=c("current"="steelblue3", "historical" = "skyblue1"),
                    labels = c("current"="Current year","historical"="Historical"),
                    drop = FALSE,
                    guide = guide_legend(nrow=1, byrow=FALSE,
                                         title.position="bottom", title.hjust=0.65)) +
  theme(legend.position="bottom",
        legend.title=element_text(size=18),
        legend.text = element_text(size=18))

A second approach to get the desired result would be to make use of a custom key_glyph which uses the default rect glyph for the fill legend in case of the historical data and and a line or path glyph for the current data.

My custom glyph function is simply a wrapper which depending on the type or actually the fill color (see the note) calls ggplot2::draw_key_rect or ggplot2::draw_key_path.

Note 1 To the best of my knowledge it's not possible to condition on type. Instead we have to condition on the assigned fill color, i.e. we have to do if (data$fill == "skyblue1") to check for type == "historical".

Note 2 As draw_key_path needs a color instead of a fill we have to manually adjust the values for color aesthetic. This could be achieved in two ways.

1. You could "hardcode" the values inside the custom function or
2. You could adjust the values of aes via guides(..., override.aes = ...)

My edited code makes use of the second approach. Additionally I set the alpha=1 for the line and adjusted the size.

library(ggplot2)

# Custom key glyph
draw_key_cust <- function(data, params, size) {
  if (data$fill == "skyblue1") {
    draw_key_rect(data, params, size)
  } else {
    # Hard code the values or make use of guides(..., override.aes(...)) (see below)
    # data$colour <- data$fill
    # data$alpha <- 1
    draw_key_path(data, params, size)
  }
}

# plot data
ggplot() +
  geom_line(df1, mapping = aes(x = as.Date(jday, origin = as.Date("2020-01-01")), y = csum, fill = "current"), colour = "steelblue3", key_glyph = "cust") +
  geom_ribbon(df2, mapping = aes(x = as.Date(jday, origin = as.Date("2020-01-01")), ymin = 0, ymax = csum, fill = "historical"), alpha = 0.5, key_glyph = "cust") +
  geom_vline(aes(xintercept = as.Date(150, origin = as.Date("2020-01-01")), colour = "Planting"), linetype = "longdash") +
  geom_vline(aes(xintercept = as.Date(180, origin = as.Date("2020-01-01")), colour = "Germination"), linetype = "longdash") +
  # geom_vline(aes(xintercept=as.Date(210, origin=as.Date("2020-01-01")), colour="Flowering"), linetype="longdash") +
  # geom_vline(aes(xintercept=as.Date(240, origin=as.Date("2020-01-01")), colour="Grain filling"), linetype="longdash") +
  # geom_vline(aes(xintercept=as.Date(270, origin=as.Date("2020-01-01")), colour="Maturity"), linetype="longdash") +
  theme_bw(base_size = 18) +
  scale_x_date(date_breaks = "15 days", date_labels = "%d/%b/%y") +
  labs(y = "Cumulative rainfall (mm)", x = "Date") +
  theme(axis.text.x = element_text(angle = 45, hjust = 1)) +
  scale_color_manual(
    name = "Phenological stages",
    breaks = c("Planting", "Germination", "Flowering", "Grain filling", "Maturity"),
    values = c(
      "Planting" = "#24CF09",
      "Germination" = "#49A012",
      "Flowering" = "#5B8817",
      "Grain filling" = "#805920",
      "Maturity" = "#A52A2A"
    )
  ) +
  theme(
    legend.position = "bottom",
    # legend.title=element_text(size=18),
    # legend.text = element_text(size=18)
  ) +
  guides(colour = guide_legend(
    nrow = 1, byrow = FALSE,
    title.position = "top", title.hjust = 0.5
  )) +
  guides(fill = guide_legend(
    nrow = 1, byrow = FALSE,
    title.position = "top", title.hjust = 0.5,
    # Use override.aes to set the aes prams for the glyph
    override.aes = list(color = c("steelblue3", NA), size = c(2, .5), alpha = c(1, .5))
  )) +
  scale_fill_manual(
    name = "Rainfall",
    values = c("current" = "steelblue3", "historical" = "skyblue1"),
    labels = c("current" = "Current year", "historical" = "Historical"),
    breaks = c("current", "historical")
  )
#> Warning: Ignoring unknown aesthetics: fill