R ggplot - plot all columns of dataframe as multiple lines with the same color

I want to plot all columns of a dataframe as separate lines in one plot. They all should have the same color (grey, dashed, in the Background). I then want to plot the mean of all values as solid fat line in the front.

My code so far works for all requirements except of the same color for all the separate lines:

  set.seed(200)                                            
  df <- data.frame(x = 1:10,
                   y1 = rnorm(5),
                   y2 = runif(5),
                   y3 = rpois(5, 1))
  
  df[,"mean"] = rowMeans(df[,c(2:4)])
  
  # Reshape data frame
  df_reshaped <- data.frame(x = df$x,                           
                            y = c(df$y1, df$y2, df$y3),
                            group = c(rep("y1", nrow(df)),
                                      rep("y2", nrow(df)),
                                      rep("y3", nrow(df))))
  
  ggplot(data = df_reshaped, aes(x=x)) +
    geom_line(
      data = df_reshaped,
      aes(y = y, col=group),
      linetype = "dashed",
      # color = "grey", # this part does not work properly
      linewidth = 0.25
    ) + 
    geom_line(
      data = df,
      aes(y = mean),
      linetype = "solid",
      color = "black",
      linewidth = 0.5
    )

My result is:

When I uncomment the line # color = "grey" I get a weird result that looks like this:

icing on the cake: Add a legend that says:

---- single values
____ mean value

Thanks, I appreciate your help!

You could add the colors using scale_color_manual with values since you assigned it to your aesthetics like this:

library(ggplot2)

ggplot(data = df_reshaped, aes(x=x)) +
  geom_line(
    data = df_reshaped,
    aes(y = y, col=group),
    linetype = "dashed",
    # color = "grey", # this part does not work properly
    linewidth = 0.25
  ) + scale_color_manual(values = rep("grey", 3)) + 
  geom_line(
    data = df,
    aes(y = mean),
    linetype = "solid",
    color = "black",
    linewidth = 0.5
  )

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^{Created on 2023-07-04 with reprex v2.0.2}

Alternative approach compiling the data into one long dataframe gives you the legend for the y values as well as the mean.

library(ggplot2)
library(dplyr)
library(tidyr)

set.seed(200)                                            
df1 <- 
  data.frame(x = 1:10,
             y1 = rnorm(5),
             y2 = runif(5),
             y3 = rpois(5, 1)) |> 
  rowwise()|>
  mutate(mean = mean(c(y1, y2, y3))) |> 
  pivot_longer(-x)



ggplot(df1, ) +
  geom_line(aes(x = x, y = value, linetype = name, linewidth = name, colour = name))+
  scale_linetype_manual(breaks = c("y1", "y2", "y3", "mean"),
                        values = c(rep("dashed", 3), "solid")) +
  scale_linewidth_manual(breaks = c("y1", "y2", "y3", "mean"),
                         values = c(rep(0.25, 3), 0.5)) +
  scale_colour_manual(breaks = c("y1", "y2", "y3", "mean"),
               values = c(rep("grey50", 3), "black"))+
  labs(y = "y",
       linetype = NULL,
       linewidth = NULL,
       colour = NULL)+
  theme_bw()

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^{Created on 2023-07-04 with reprex v2.0.2}