ggplot2 - how to add a break between dotted lines in legend?

The following code produces two histograms. I'm wondering whether there's any simple way to create a break in the dotted lines in the legend for these histograms. See the legend in the attached image - I want the dotted lines not to run into each other?

#make low cost histogram

PLOT1 <- LCset1 %>% 
  ggplot(aes(x=donation, fill=lowmonetaryamount))+
  coord_cartesian(xlim = c(1, NA)) +
  geom_histogram(binwidth=1,aes(y = (..count..)/sum(..count..)),col=I("black"))+
  scale_color_grey()+scale_fill_grey(start = .85,
                                     end = .85,) +
  theme_linedraw()+
  guides(fill = "none", cols='none')+
  geom_vline(aes(xintercept=10, size='\nLow cost\n'),
             color="black", linetype=5)+
  geom_vline(aes(xintercept=50, size='\nHigh cost\n'),
             color="black", linetype="dotted")+
  scale_size_manual(values = c(.5, 0.5), breaks = c('\nLow cost\n','\nHigh cost\n'), guide=guide_legend(title = "", override.aes = list(linetype=c(5,3), color=c('black', 'black'))))+
  scale_y_continuous(labels=scales::percent, limits = c(0, .015))+
  scale_x_continuous(breaks = c(seq(0,50,10), 55), labels = c("","$10","$20","$30","$40","$50","      >$50"), limits = c(0, 60))+
  ggtitle("\n% donating each non-zero amount when low cost is $10")+
  theme(plot.title = element_text(hjust = 0.5))+
  xlab(NULL)+
  ylab(NULL)+
  theme(plot.title = element_text(size=13))+
  theme(
    # other theme parameters
    legend.key.size = unit(2, "lines"),     
    legend.key.width = unit(1, "lines"),
    legend.text = element_text(size=12),
    axis.text = element_text(size=12),
    legend.key.height = unit(0.5, "lines")
  )

#make high cost histogram 

PLOT2 <- HCset1 %>% 
  ggplot(aes(x=donation, fill=lowmonetaryamount))+
  coord_cartesian(xlim = c(1, NA)) +
  geom_histogram(binwidth=1,aes(y = (..count..)/sum(..count..)),col=I("black"))+
  scale_color_grey()+scale_fill_grey(start = .85,
                                     end = .85,) +
  theme_linedraw()+
  guides(fill = "none", cols='none')+
  geom_vline(aes(xintercept=10, size='\nLow cost\n'),
             color="black", linetype=5)+
  geom_vline(aes(xintercept=50, size='\nHigh cost\n'),
             color="black", linetype="dotted")+
  scale_size_manual(values = c(.5, 0.5), breaks = c('\nLow cost\n','\nHigh cost\n'), guide=guide_legend(title = "", override.aes = list(linetype=c(5,3), color=c('black', 'black'))))+
  scale_y_continuous(labels=scales::percent, limits = c(0, .015))+
  scale_x_continuous(breaks = c(seq(0,50,10), 55), labels = c("","$10","$20","$30","$40","$50","      >$50"), limits = c(0, 60))+
  ggtitle("% donating each non-zero amount when high cost is $50")+
  theme(plot.title = element_text(hjust = 0.5))+
  xlab(NULL)+
  ylab(NULL)+
  theme(plot.title = element_text(size=13))+
  theme(
    # other theme parameters
    legend.key.size = unit(2, "lines"),     
    legend.key.width = unit(1, "lines"),
    legend.text = element_text(size=12),
    axis.text = element_text(size=12),
    legend.key.height = unit(0.5, "lines")
  )

#combine two histograms

plot<- ggarrange(PLOT1, PLOT2,ncol=1,label.x=1,common.legend = TRUE, legend="right")
plots <- grid.arrange(plot,top=textGrob("Histograms for the 40.1% of the total sample whose last donation was between $10.00 and $49.99",gp=gpar(fontsize=14,fontface="bold")))

ggsave("plots_1.png", plots, width = 10, height = 8)

I tried adding legend.spacing.y to the theme section, but this only moves the entire legend down. I'd like to create a distinct break so that the two dotted lines don't touch each other.

The trick to making legend.spacing.y work is the byrow=TRUE param on guide_legend

library(ggplot2)
ggplot(mtcars, aes(y = factor(cyl), fill = factor(cyl))) +
  geom_bar() +
  theme(legend.spacing.y = unit(2.0, 'cm')) +
  guides(fill = guide_legend(byrow = TRUE))