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))
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On Wed, 10 Jan 2024 19:13:19 -0500 Deepankar Basu <basu.15 at gmail.com> wrote:> 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))My solution would be to use base graphics and avoid the obfuscation induced by ggplot(). The graphics produced by ggplot() are devilish pretty, but ggplot() is devilish difficult to work with. IMHO you need an IQ of at least 200 (which leaves me about 150 points short!) to be able to cope with its intricate and unintuitive syntax. I believe that what you want to do is triv in base graphics, but I must confess that I have not gone through the details. cheers, Rolf Turner -- Honorary Research Fellow Department of Statistics University of Auckland Stats. Dep't. (secretaries) phone: +64-9-373-7599 ext. 89622 Home phone: +64-9-480-4619
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]] > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.
? Wed, 10 Jan 2024 19:13:19 -0500 Deepankar Basu <basu.15 at gmail.com> ?????:> 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?Can you use the information about the function (i.e. the myg object), or are you restricted to the d2 data.frame? Both ggplot and the core graphics use the algorithm from the contourLines() function in order to draw the contour lines. (Lattice does the same, but ports the function from C to R.) This function returns a list of levels combined with vectors of `x` and `y` coordinates for drawing the contour. If you can use the myg function, you can differentiate it numerically or symbolically, process the contourLines() output, evaluate the gradient at one point per contour and draw additional arrows there. If you are limited to the tabulated function values from the d2 data.frame, you have a more complicated problem on your hands. You can either locate grid points closest to the contour segments at both sides of the segment, determine the direction of growth, and draw an arrow perpendicular to the contour segment (not equal to the gradient), or find a way to estimate the gradient at arbitrary points between the grid (e.g. by using a Taylor series stencil) and then go to the previous paragraph. -- Best regards, Ivan
Something like this shodld worx. You will need to fiddle around with the actual co-ordinates etc. I just stuck an arrow in what seemed like a handy place On Wed, 10 Jan 2024 at 19:13, Deepankar Basu <basu.15 at gmail.com> wrote:> 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]] > > ______________________________________________ > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >-- John Kane Kingston ON Canada [[alternative HTML version deleted]]