Potential bug in `create_coverageplot`

[helen-zhu]

  histogram_data <- histogram_obj$histogram_data
  # choosing the midpoint of the start/end as the label
  plotting_data <- data.frame(
    "dens" = histogram_data,
    "labels_x" = seq(1, length(histogram_data), 1),
    "dist" = "coverage"
  )

In the above case, labels_x should probably be the midpoints

Also, it seems reasonable to add a function to generate the fitted density of the distributions