How to align legend elements in the middle when they are on two rows using ggplot2?

Question

In the MWE below, the caption appears separated into two rows. I want the item corresponding to gear 5 to be aligned in the middle in relation to gears 3 and 4 above it.

How can I do this?

library(ggplot2)

ggplot(data = mtcars,
       aes(x = disp,
           y = qsec,
           shape = as.factor(gear),
           color = as.factor(gear))) +
  geom_line() +
  geom_point() +
  theme(legend.position = "bottom",
        legend.title = element_blank()) +
  guides(shape=guide_legend(nrow = 2,
                            byrow = TRUE),
         color = guide_legend(nrow = 2,
                              byrow = TRUE))

[EDIT]

I want the plot to look like this: