How can I draw this graph using iris data?

For 1), yes, the lines are ugly. I don't know what's causing it, but since each bar is a rectangle, I think it must be a graphical glitch. You can avoid this by setting the color of the histogram to be the same as the fill.

For 2), I'm going to play the adventure game, i.e. instead of giving the answer you want to hear (ie how to do these things in the grid), I'll give you an answer that I think you need to hear.

Assuming your goal is to display these histograms as marginal histograms on the main panel, you can easily achieve something similar in patchwork.

library(ggplot2)
library(patchwork)
#> Warning: package 'patchwork' was built under R version 3.6.3

a <- ggplot(data = iris,
            aes(x=Sepal.Length, y=Sepal.Width)) + 
  geom_point()

b <- ggplot(data = iris,
            aes(x = Sepal.Length)) + 
  geom_histogram()


c <- ggplot(data = iris,
            aes(x = Sepal.Width)) + 
  geom_histogram() +
  coord_flip()

b + plot_spacer() + a + c + 
  plot_layout(nrow = 2, ncol = 2, widths = c(1, 0.5), heights = c(0.5, 1))
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2020-04-18 by the reprex package (v0.3.0)}

You can also ignore axes for edge plots by setting breakpoints and names:

b + scale_x_continuous(breaks = NULL, name = "") + 
  plot_spacer() +
  a + 
  c + scale_x_continuous(breaks = NULL, name = "") +
  plot_layout(nrow = 2, ncol = 2, widths = c(1, 0.5), heights = c(0.5, 1))