Hi all,

I produced a graph using geom_smooth() function and I want to identify the point that there is a change in trend from upwards to downwards. Does someone has an idea how this happens?

Many thanks in advance.

Panos

I don't think `geom_smooth()`

can give you that directly. Under the hood, `geom_smooth()`

is actually using a type of `loess()`

smoothing, so you could use that directly:

```
library(tidyverse)
dat <- tibble(x = 1:100,
y = ifelse(x<50, 2*x, -2*x + 200) + rnorm(100, sd = 10))
ggplot(dat, aes(x, y)) +
geom_point() +
geom_smooth()
#> `geom_smooth()` using method = 'loess' and formula = 'y ~ x'
```

```
mod <- loess(y ~ x, data = dat)
predict(mod, 1:100) |>
diff() |>
sign() |>
diff()
#> [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> [26] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -2 0 0
#> [51] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> [76] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
```

^{Created on 2023-06-19 with reprex v2.0.2}

But for more advanced methods, I suggest you look up "R time series changepoint detection", there is a lot of resources that exist!

Thanks Alex! However still I cannot find the point where the trend is changing

Of the steps to follow that Alex laid out, which one trips you up ?

What Alex suggested leads to a graph with a smooth_line. However I want (say in his graph) to see the coordinates where the smooth_line changes inclination (from upwards to dowanwards)

Alex's example went as far as to detect the difference in sign as x is traversed. we can find the location by adding a little more

```
predict(mod, 1:100) |>
diff() |>
sign() |>
diff() |> which.min()
```

this gives me '49'

the 49th position of 1:100 is itself '49'

Here's a further example adapted from Alex, where the correspondence with x is less obvious, but still retreivable.

```
library(tidyverse)
dat <- tibble(x = 25:60,
y = ifelse(x<50, 2*x, -2*x + 200) + rnorm(36, sd = 10))
ggplot(dat, aes(x, y)) +
geom_point() +
geom_smooth()
#> `geom_smooth()` using method = 'loess' and formula = 'y ~ x'
mod <- loess(y ~ x, data = dat)
predict(mod, 25:60) |>
diff() |>
sign() |>
diff() |> which.min()
#24
(25:60)[24]
# x=48
```

in this example x is from 25:60 , the point of sign change is the 24th entry of x, which is the value x=48

I suppose your question may be simply how to annotate a chart ?

```
+ geom_vline(xintercept = 48)
```

will add a vertical line at the x value you choose

My question includes how to annotate the chart but I don't want all points to be shown. Moreover, how do you know that xintercept=48? It makes sense after the piece of work that you added.

Many thanks for your help!

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