Add a common legend

I was trying to do a multiplot with ggplot2. This was my initial code

nucmer_s1 <- ggarrange(eight_uniform, ten_uniform, twelve_uniform, fourteen_uniform, sixteen_uniform, 
                       ncol=3, nrow=2, common.legend = TRUE, legend="bottom")

getting this error

Error in plot$scales : $ operator is invalid for atomic vectors

then.

annotate_figure(nucmer_s1, 
                top = text_grob("Genomas validados con distribución de datos equilibrada",
                color = "black", face = "bold", size = 12))

however I obtain the graphic But I need to put a title in the each plot a title so I changed to this one

nucmer_s1 <-grid.arrange(
  eight_uniform + ggtitle("8 genomas"), 
  ten_uniform +  ggtitle("10 genomas"), 
  twelve_uniform + ggtitle("12 genomas"), 
  fourteen_uniform + ggtitle("14 genomas"), 
  sixteen_uniform + ggtitle("16 genomas"), 
  ncol=3, nrow=2, common.legend = TRUE, legend="bottom")

but I got

Error in gList(list(grobs = list(list(x = 0.5, y = 0.5, width = 1, height = 1, :
only 'grobs' allowed in "gList"
Además: Warning messages:
1: In grob$wrapvp <- vp : Realizando coercion de LHD a una lista
2: In grob$wrapvp <- vp : Realizando coercion de LHD a una lista

so I erase the common.legend part and got this plot

So I have two questions:

Is there a way to put a title in each plot with the grey box without using facet_grid (cause I don't have that info in the data)? and
Is there any way to put the legend in the blank side of a multi-plot?

Thank so much for your help

lemon or cowplot packages have really nice built-in functions to deal with shared legend between plots

example from lemon package

library(ggplot2)
library(grid)
library(gtable)
library(lemon)

dsamp <- diamonds[sample(nrow(diamonds), 1000), ]
d <- ggplot(dsamp, aes(carat, price)) +
  geom_point(aes(colour = clarity)) +
  theme(legend.position = c(0.06, 0.75))

d3 <- d + 
  facet_wrap(~cut, ncol=3) + 
  scale_color_discrete(guide=guide_legend(ncol=3))

# Use gtable_show_names to display the names of the facetted panels
gtable_show_names(d3)

# So to place the legend in a specific panel, give its name:
reposition_legend(d3, 'center', panel='panel-3-2')

example from cowplot package

library(cowplot)

# 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
           )

# extract the legend from one of the plots
# (clearly the whole thing only makes sense if all plots
# have the same legend, so we can arbitrarily pick one.)
legend <- get_legend(p1)

# add the legend to the row we made earlier. Give it one-third of the width
# of one plot (via rel_widths).
p <- plot_grid( prow, legend, rel_widths = c(3, .3))
p