I am trying to reproduce this kind of Figure, with two densities, a first one pointing upwards and a second one pointing downwards. I would also like to have some blank space between the two densities.

[![enter image description here](https://i.stack.imgur.com/owkA3.png)](https://i.stack.imgur.com/owkA3.png)

Here is the code I am currently using.

```
library(hrbrthemes)
library(tidyverse)
library(RWiener)

# generating data
df <- rwiener(n = 1e2, alpha = 2, tau = 0.3, beta = 0.5, delta = 0.5)

df %>%
  ggplot(aes(x = q) ) +        
  geom_density(
    data = . %>% filter(resp == "upper"),
    aes(y = ..density..),
    colour = "steelblue", fill = "steelblue",
    outline.type = "upper", alpha = 0.8, adjust = 1, trim = TRUE
    ) +
  geom_density(
    data = . %>% filter(resp == "lower"),
    aes(y = -..density..), colour = "orangered", fill = "orangered",
    outline.type = "upper", alpha = 0.8, adjust = 1, trim = TRUE
    ) +
  # stimulus onset
  geom_vline(xintercept = 0, lty = 1, col = "grey") +
  annotate(
    geom = "text",
    x = 0, y = 0,
    # hjust = 0,
    vjust = -1,
    size = 3, angle = 90,
    label = "stimulus onset"
    ) +
  # aesthetics
  theme_ipsum_rc(base_size = 12) +
  theme(axis.text.y = element_blank() ) +
  labs(x = "Reaction time (in seconds)", y = "") +
  xlim(0, NA)

```

Which results in something like...

[![enter image description here](https://i.stack.imgur.com/EndQR.png)](https://i.stack.imgur.com/EndQR.png)

How could I add some vertical space between the two densities to reproduce the above Figure?