Question: ```
ggplot(all, aes(x=area, y=nq)) +
  geom_point(size=0.5) +
  geom_abline(data = levelnew, aes(intercept=log10(exp(interceptmax)), slope=fslope)) + #shifted regression line
  scale_y_log10(labels = function(y) format(y, scientific = FALSE)) + 
  scale_x_log10(labels = function(x) format(x, scientific = FALSE)) + 
  facet_wrap(~levels) +
  theme_bw() +
  theme(panel.grid.major = element_line(colour = "#808080"))

```

And I get this figure

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

Now I want to add **one geom\_line** to one of the facets. Basically, I wanted to have a dotted line (Say x=10,000) in only the **major** panel. How can I do this?

Answer: 
I don't have your data, so I made some up:

```
df <- data.frame(x=rnorm(100),y=rnorm(100),z=rep(letters[1:4],each=25))

ggplot(df,aes(x,y)) +
  geom_point() +
  theme_bw() +
  facet_wrap(~z)

```

[![enter image description here](https://i.stack.imgur.com/uCOR9.png)](https://i.stack.imgur.com/uCOR9.png)

To add a vertical line at `x = 1` we can use `geom_vline()` with a dataframe that has the same faceting variable (in my case `z='b'`, but yours will be `levels='major'`):

```
ggplot(df,aes(x,y)) +
  geom_point() +
  theme_bw() +
  facet_wrap(~z) +
  geom_vline(data = data.frame(xint=1,z="b"), aes(xintercept = xint), linetype = "dotted")

```

[![enter image description here](https://i.stack.imgur.com/0uBOB.png)](https://i.stack.imgur.com/0uBOB.png)
Another way to express this which is possibly easier to generalize (and formatting stuff left out):

```
ggplot(df, aes(x,y)) +
  geom_point() + 
  facet_wrap(~ z) +
  geom_vline(data = subset(df, z == "b"), aes(xintercept = 1))

```

The key things being: facet first, then decorate facets by subsetting the original data frame, and put the details in a new `aes` if possible. Other examples of a similar idea:

```
ggplot(df, aes(x,y)) +
  geom_point() + 
  facet_wrap(~ z) +
  geom_vline(data = subset(df, z == "b"), aes(xintercept = 1)) + 
  geom_smooth(data = subset(df, z == "c"), aes(x, y), method = lm, se = FALSE) +
  geom_text(data = subset(df, z == "d"), aes(x = -2, y=0, label = "Foobar"))

```