Standalone legend in ggpairs - r

Question

How can I include a legend inside one of the empty panels of the following matrix plot?
I have color coded different regression lines in the plots. I need a legend based on color.
I believe this answer comes closest to answer my question, yet I do not know how exactly to modify my code to get a legend based on color for different regression lines.
As for the background of the code, I am trying to study different robust and non-robust regression methods applied to multivariate data with and without outliers.
library(ggplot2)
library(GGally)
library(MASS)
library(robustbase)
## Just create data -- you can safely SKIP this function.
##
## Take in number of input variables (k), vector of ranges of k inputs
## ranges = c(min1, max1, min2, max2, ...) (must have 2k elements),
## parameters to create data (must be consistent with the number of
## input variables plus one), parameters are vector of linear
## coefficients (b) and random seed (seed), number of observations
## (n), vector of outliers (outliers)
##
## Return uncontaminated dataframe and contaminated dataframe
create_data <- function(k, ranges, b, seed = 6, n,
outliers = NULL) {
x <- NULL # x: matrix of input variables
for (i in 1:k) {
set.seed(seed^i)
## x <- cbind(x, runif(n, ranges[2*i-1], ranges[2*i]))
x <- cbind(x, rnorm(n, ranges[2*i-1], ranges[2*i]))
}
set.seed(seed - 2)
x_aug = cbind(rep(1, n), x)
y <- x_aug %*% b
y_mean = mean(y)
e <- rnorm(n, 0, 0.20 * y_mean) # rnorm x
y <- y + e
df <- data.frame(x = x, y = y)
len <- length(outliers)
n_rows <- len %/% (k+1)
if (!is.null(outliers)) {
outliers <- matrix(outliers, n_rows, k+1, byrow = TRUE)
df_contamin <- data.frame(x = rbind(x, outliers[,1:k]), y = c(y, outliers[,k+1]))
} else {
df_contamin <- df
}
dat <- list(df, df_contamin)
}
# plot different regression models (some are robust) for two types of
# data (one is contaminated with outliers)
plot_models <- function(data, mapping, data2) {
cb_palette <- c("#999999", "#E69F00", "#56B4E9", "#009E73", "#F0E442", "#0072B2", "#D55E00", "#CC79A7")
## 1.grey, 2.light orange, 3.light blue, 4.green, 5.yellow, 6.blue, 7.red, 8.purple
plt <- ggplot(data = data, mapping = mapping) +
geom_point() +
theme_bw() +
geom_smooth(method = lm, formula = y ~ x, data = data2, color = cb_palette[3], se = FALSE) +
geom_smooth(method = lm, formula = y ~ x, color = cb_palette[7], se = FALSE) +
geom_smooth(method = rlm, formula = y ~ x, color = cb_palette[4], se = FALSE) +
geom_smooth(method = lmrob, formula = y ~ x, color = cb_palette[1], se = FALSE)
plt
}
# trim the upper and right panels of plots
trim_gg <- function(gg) {
n <- gg$nrow
gg$nrow <- gg$ncol <- n-1
v <- 1:n^2
gg$plots <- gg$plots[v > n & v%%n != 0]
gg$xAxisLabels <- gg$xAxisLabels[-n]
gg$yAxisLabels <- gg$yAxisLabels[-1]
gg
}
dat <- create_data(3, c(1, 10, 1, 10, 1, 10), c(5, 8, 6, 7), 6, 20, c(30, 30, 50, 400))
df <- dat[[1]]
df_contamin <- dat[[2]]
## Note that plot_models is called here
g <- ggpairs(df_contamin, columns = 1:4, lower = list(continuous = wrap(plot_models, data2 = df)), diag = list(continuous = "blankDiag"), upper = list(continuous = "blank")) #, legend = lgd)
gr <- trim_gg(g)
print(gr)
Created on 2019-10-09 by the reprex package (v0.3.0)
Sorry for the long code, but most probably only the plot_models function and the line where ggpairs is called need to be modified.
I want to get a legend in the blank upper half of the plots. It may be done by somehow tweaking the plot_models function, setting the mapping in ggpairs to color using ggplot2::aes_string, and using getPlot and putPlot of the GGally package. But I can't wrap my head around how to do it exactly.