pr10

step 1:

library(ggplot2)

step 2: Define function

plot_density_by_group <- function(data,continuous_var,group_var,fill_colors = NULL){
  if(!(continuous_var %in% names(data)) || !(group_var %in% names(data)))
  {
    stop("Invaild column names.Make sure both variable exist in the dataset.")
  }
  
  p <- ggplot(data,aes_string(x = continuous_var,color = group_var,fill = group_var))+
    geom_density(alpha=0.4)+
    labs(title = paste("Density plot of",continuous_var,"by",group_var),
         x = continuous_var,
         y = "Density")+
    theme_minimal()
  if(!is.null(fill_colors)){
    p <- p + scale_fill_manual(values = fill_colors)+
      scale_color_manual(values = fill_colors)
  }
  return(p)
}

# Basic usage
plot_density_by_group(iris,"Sepal.Length","Species")

Warning: `aes_string()` was deprecated in ggplot2 3.0.0.
ℹ Please use tidy evaluation idioms with `aes()`.
ℹ See also `vignette("ggplot2-in-packages")` for more information.

custom_colors <- c("setosa" = "steelblue",
                   "versicolor" = "forestgreen",
                   "virginica"= "darkorange")
plot_density_by_group(iris,"Petal.Length","Species",fill_colors = custom_colors)

custom_colors <- c("setosa" = "steelblue",
                   "versicolor" = "forestgreen",
                   "virginica"= "darkorange")
plot_density_by_group(iris,"Sepal.Length","Species",fill_colors = custom_colors)

custom_colors <- c("setosa" = "steelblue",
                   "versicolor" = "forestgreen",
                   "virginica"= "darkorange")
plot_density_by_group(iris,"Petal.Width","Species",fill_colors = custom_colors)

custom_colors <- c("setosa" = "steelblue",
                   "versicolor" = "forestgreen",
                   "virginica"= "darkorange")
plot_density_by_group(iris,"Sepal.Width","Species",fill_colors = custom_colors)