[R] arrow on contour line

[Lennart Kasserra]

Hi,

If I understand your intentions correctly, the simplest way would be to 
manually insert arrows using `geom_segment()` with the `arrow` argument:

```

ggplot(data = d1, aes(x=X1,y=X2,z=Z))+
   stat_contour(breaks = c(2)) +
   stat_contour(data=d2, aes(x=X1,y=X2,z=Z), breaks=c(6)) +
   geom_segment(
     aes(x = 1.25, y = 1.50, xend = 1.4, yend = 1.325),
     arrow = arrow(length = unit(0.5, "cm"))
   ) +
   geom_segment(
     aes(x = 1.6, y = 1.95, xend = 1.8, yend = 1.75),
     arrow = arrow(length = unit(0.5, "cm"))
   )

```

If your actual data is organized differently, you may also be able to do 
this in a more elegant programmatic way. Here is a StackOverflow 
conversation about this exact topic, one Google search away: 
https://stackoverflow.com/questions/38008863/how-to-draw-a-nice-arrow-in-ggplot2

Have a nice day,

Lennart Kasserra

Am 11.01.24 um 01:13 schrieb Deepankar Basu:
> Hello,
>
> I am drawing contour lines for a function of 2 variables at one level of
> the value of the function and want to include a small arrow in any
> direction of increase of the function. Is there some way to do that?
>
> Below is an example that creates the contour lines. How do I add one small
> arrow on each line in the direction of increase of the function (at some
> central point of the contour line)? Any direction will do, but perhaps the
> direction of the gradient will be the best.
>
> Thanks in advance.
> DB
>
> --------------------------------------------
>
> library(tidyverse)
>
> x <- seq(1,2,length.out=100)
> y <- seq(1,2,length.out=100)
>
> myf <- function(x,y) {x*y}
> myg <- function(x,y) {x^2 + y^2}
>
> d1 <- expand.grid(X1 = x, X2 = y) %>%
>    mutate(Z = myf(X1,X2)) %>%
>    as.data.frame()
>
> d2 <- expand.grid(X1 = x, X2 = y) %>%
>    mutate(Z = myg(X1,X2)) %>%
>    as.data.frame()
>
> ggplot(data = d1, aes(x=X1,y=X2,z=Z))+
>    stat_contour(breaks = c(2)) +
>    stat_contour(data=d2, aes(x=X1,y=X2,z=Z), breaks=c(6))
>
> 	[[alternative HTML version deleted]]
>
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