Plot a rarefaction curve to determine sequence saturation
`plot_signatures_rarefaction()` produces a ggplot2 plot from the tidy data frame output by `from_signatures_to_rarefaction_df()`. Since the rarefaction curves may contain many dense points, the `fraction_of_points_to_plot` argument reduces the total number of points rendered which decreases the functions run time. Rarefaction curves that end with a shallow slow likely represent samples that were sequenced deeply enough to capture the k-mer diversity in the sequence.
A tidy data frame returned by `from_signatures_to_rarefaction_df()`. Must contain the columns `num_kmers_sampled`, `num_kmers_observed`.
Integer. 1 every `fraction_of_points_to_plot` will be plotted.