将共享图例与打印网格的中心对齐(使用cowplot)

https://cran.r-project.org/web/packages/cowplot/vignettes/shared_legends.html

library(ggplot2)
library(cowplot)
#down-sampled diamonds data set
dsamp <- diamonds[sample(nrow(diamonds), 1000), ]

# Make three plots.
# We set left and right margins to 0 to remove unnecessary spacing in the
# final plot arrangement.
p1 <- qplot(carat, price, data=dsamp, colour=clarity) +
  theme(plot.margin = unit(c(6,0,6,0), "pt"))
p2 <- qplot(depth, price, data=dsamp, colour=clarity) +
  theme(plot.margin = unit(c(6,0,6,0), "pt")) + ylab("")
p3 <- qplot(color, price, data=dsamp, colour=clarity) +
  theme(plot.margin = unit(c(6,0,6,0), "pt")) + ylab("")

# arrange the three plots in a single row
prow <- plot_grid( p1 + theme(legend.position="none"),
           p2 + theme(legend.position="none"),
           p3 + theme(legend.position="none"),
           align = 'vh',
           labels = c("A", "B", "C"),
           hjust = -1,
           nrow = 1
           )
legend_b <- get_legend(p1 + theme(legend.position="bottom"))

# add the legend underneath the row we made earlier. Give it 10% of the height
# of one plot (via rel_heights).
p <- plot_grid( prow, legend_b, ncol = 1, rel_heights = c(1, .2))
p

此示例显示了一个绘图,其中图例绘制与网格左下角对齐。 然而,它过去是不同的,因为图例然后被绘制成与绘图的底部中心对齐。下面是几个月前由我的个人代码生成的示例。 https://s1.postimg.org/8pf2en1zen/Untitled.png

在任意一个包中发生未知数量的更改后重新运行我的旧代码,会产生一个左下角对齐的图例(也如教程中所示,上面的第三个图): https://s1.postimg.org/3agjw7n9gf/Untitled2.png

问题是如何调整代码以绘制与底部中心对齐的图例。