Additional adjustments for the legend, I guess.
fill (instead of col) is in there as it is the type of graph you are using.
The guides (the axes and legends) help readers interpret your plots. Guides are mostly controlled via the scale (e.g. with the limits , breaks , and labels arguments), but sometimes you will need additional control over guide appearance. Use guides() or the guide argument to individual scales along with guide_colourbar() or guide_legend() .
From here: https://ggplot2.tidyverse.org/reference/index.html