How to modify ggplot2 legend keys?

For reference, I am posting the solution.

library(ggplot2)

ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species, fill = Species)) +
  geom_point(key_glyph = "rect") +
  theme(
    legend.position = "top",
    legend.title = element_blank()
  ) +
  guides(
    color = guide_legend(
      label.position = "top",
      keywidth = unit(2, "cm"), # This is not giving me what I was expecting.
    )
  )

^{Created on 2020-07-23 by the reprex package (v0.3.0)}

While @Ian's answer works, there's a far simpler way, which is to define the legend key glyph you want to use right in the geom_point() call. The important point to note is that if we specify the key glyph should be a rect, we need to provide the fill aesthetic (or you'll just have empty rectangles for the glyphs):

ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
  geom_point(aes(fill=Species), key_glyph='rect') +
  theme(
    legend.position = "top",
    legend.title = element_blank()
  )

You should be able to adjust the key dimensions from there via guides() or theme() changes to suit your needs.

Using inspiration from this answer from baptiste, one approach might be to replace the GeomPoint$draw_key function with GeomBar$draw_key.

GeomPoint$draw_key <- GeomBar$draw_key
ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, color = Species)) +
  geom_point() +
  scale_fill_identity() +
  theme(
    legend.position = "top",
    legend.title = element_blank()
  ) +
  guides(
    color = guide_legend(
      label.position = "top",
      keywidth = unit(2, "cm")
    )
  )