R/plot_residuals.R

In PatrickEslick/BrasiliaWorkshop: Practice For Our Workshop

#' Make a plot of residuals vs fitted values
#' 
#' Plot residuals against fitted values to check for 
#' homoscedasticity. 
#' 
#' @param model an lm object. The function will extract
#'   residuals and fitted values from the model object.
#' 
#' @return a ggplot object. A plot showing residual on the
#'   y-axis and fitted value on the x-axis.
#' 
#' @import ggplot2
#' 
#' @export
#' 
#' @examples 
#' 
#' model <- lm(mpg~wt, data = mtcars)
#' plot <- plot_residuals(model)
#' plot

plot_residuals <- function(model) {
  
  # We want to use ggplot2 to make a plot of model residual on the y-axis, 
  # and fitted values on the x-axis.
  
  # The first step will be to extract the fitted values and residuals from
  # the model. We can use the `fitted.values()` function and the `residuals()` 
  # function
  
  # ?fitted.values
  # ?residuals
  
  fitted_values <- fitted.values(model)
  resid <- residuals(model)
  
  # Make a data frame with these two vectors using data.frame() 
  
  residual_df <- data.frame(
    fitted_values = fitted_values,
    resdiduals = resid
  )
  
  # Now use the data frame with ggplot2
  
  plot <- ggplot(data = residual_df) + # Fill in the 'data' argument in this function
    geom_point(mapping = aes(x = fitted_values, y = resdiduals)) + # Fill in the 'mapping' argument in this function
    geom_hline(yintercept = 0) +
    xlab("Fitted values") +
    ylab("Residual") +
    ggtitle("Residual vs fitted value") +
    theme_bw()
  
  # We need to return our plot
  return(plot)
  
}