ggplot melted data sharing common aesthetics

I need to plot a graph from a melted data.table that mixes three geoms: geom_line, geom_col and geom_point. geom_line and geom_point share a the color aesthetics:

set.seed(1)
time <- 1:10
mag  <- 10:20
q_mag <- c("up", "up", "down")
l_a  <- sample(1:12, 10)
l_b  <- sample(8:20, 10)
p_a  <- sample(1:12, 10)
p_b  <- sample(8:20, 10)

dt <- data.table(time, mag, q_mag, l_a, l_b, p_a, p_b)
dt <- melt(dt, measure.vars = c("mag", "l_a", "l_b", "p_a", "p_b"))


ggplot(data = dt, aes (x = time)) +
  geom_col( data = dt[variable %in% "mag"]
          , aes( y    = value
               , fill = q_mag)) +
  
  scale_fill_grey(start = .4) +
  
  geom_line( data = dt[variable %in% c("l_a", "l_b")]
            ,aes( y = value
                , color = variable)
            ,size =1) +
  
  geom_point( data = dt[variable %in% c("p_a", "p_b")]
             ,aes( y = value
                  ,color = variable
                  ,size = variable
                  ,shape = variable)) +

  scale_shape_manual( name = element_blank() 
                    , labels = c("Point A", "Point B")
                    , values = c("circle", "cross")
  )+    
  
  scale_size_manual( name = element_blank() 
                   , labels = c("Point A", "Point B")
                   , values = c(4,2)
  )+    
  
  scale_color_manual( name = element_blank()
                    , labels = c("Line A", "Line B", "Point A", "Point B")
                    , values = c("red", "green", "red", "blue")
  )

This code plots the graph as expected:

However, I would like to have the legend presented in another way:

Point A as a large red circle,
Point B as a small blue cross,
Line A as a straight red line (no point), and
Line B as a straight green line (no point).

How do I accomplish the desired legend?

Solution

This could be easily achieved via the ggnewscale package which allows for multiple color scales:

library(ggplot2)
library(data.table)
library(reshape2)
library(ggnewscale)

set.seed(1)
time <- 1:10
mag <- 10:20
q_mag <- c("up", "up", "down")
l_a <- sample(1:12, 10)
l_b <- sample(8:20, 10)
p_a <- sample(1:12, 10)
p_b <- sample(8:20, 10)

dt <- data.table(time, mag, q_mag, l_a, l_b, p_a, p_b)
dt <- melt(dt, measure.vars = c("mag", "l_a", "l_b", "p_a", "p_b"))

dt <- as.data.table(dt)

ggplot(data = dt, aes(x = time, y = value)) +
  geom_col(
    data = dt[variable %in% "mag"],
    aes(
      fill = q_mag
    )
  ) +
  geom_point(
    data = dt[variable %in% c("p_a", "p_b")],
    aes(
      color = variable,
      size = variable,
      shape = variable
    )
  ) +
  scale_fill_grey(start = .4) +
  scale_shape_manual(
    name = element_blank(),
    labels = c("Point A", "Point B"),
    values = c("circle", "cross")
  ) +
  scale_size_manual(
    name = element_blank(),
    labels = c("Point A", "Point B"),
    values = c(4, 2)
  ) +
  scale_color_manual(
    name = element_blank(),
    labels = c("Point A", "Point B"),
    values = c("red", "blue")
  ) + 
  new_scale_color() +
  geom_line(
    data = dt[variable %in% c("l_a", "l_b")],
    aes(
      color = variable
    ), size = 1
  ) +
  scale_color_manual(
    name = element_blank(),
    labels = c("Line A", "Line B"),
    values = c("red", "green")
  )