有两种可能的方法可以考虑，都假设你的likert图被称为p。

p <- likert.density.plot(likertdf, facet=FALSE, bw=0.5)

方法一：从p中获取计算出的密度数据集并自行绘制

p[["data"]] %>%
  ggplot(aes(x = x, y = y, fill = Item)) +
  geom_polygon(color = "black", linewidth = 2)  +

# everything below here attempts to replicate likert.density.plot's default
# appearance; you can change them to suit your needs

  labs(fill = "Item", x = "", y = "") +
  scale_x_continuous(breaks = 1:likertdf$nlevels, 
                     labels = likert:::label_wrap_mod(paste0(levels(likertdf$items[, 1]), 
                                                             " (", 
                                                             1:likertdf$nlevels,
                                                             ")"), 
                                                      width = 10)) +
  theme(axis.text.y = element_blank(),
        axis.ticks.y = element_blank())

方法二：修改/删除p中的违规元素

p$layers[[1]]$aes_params$alpha <- 1        # set 1st layer (geom_polygon)'s alpha as 1
p$layers[[1]]$aes_params$colour <- "black" # add black outline
p$layers[[1]]$aes_params$linewidth <- 2    # specify thickness of black outline
p$layers[[3]] <- NULL # remove 3rd layer (geom_path)
p$layers[[2]] <- NULL # remove 2nd layer (geom_vline)

plot(p)

两种方法的结果：