gg_color_swatches: Generate a set of color swatches to be colored via ggplot2's scale_fill_* mechanism
Description
This function is similar to palette_plot, with two main differences: First, unlike
palette_plot, gg_color_swatches cannot label the colors with their hex code. Second, the colors don't
need to be provided as an argument, the resulting plot can be directly styled by adding
a scale_fill_* expression. Thus, this function is particularly useful to visualize
existing ggplot2 color scales.