 # transforming coordinate system

Hi,

I've got some data that roughly follows some `f(x)` . To now show the difference against that function, I'd like to transform the coordinate system, so that the `f(x)` line becomes the horizontal x-axis, and the (previously horizontal) y-grid lines become curves that are labelled on the right and left side of the diagram (like height lines on a map).

I looked at `coord_trans` , but that only allows to give single-parameter functions for x and y transformations; I'd like some way to specify eg. `1-exp(-x)` should become the x-axis line, so the new y-coordinates depend on x as well.
The graph data (points and/or lines) and grid should get transformed as well, of course.

Is that already possible? Would it be enough to pass the x coordinate to the y transformer function as well to use it for coordinate translation?

Thank you for any help!

Could you please turn this into a self-contained reprex (short for reproducible example) or even a little diagram, since this question is more theoretical? It will help us help you if we can be sure we're all working with/looking at the same stuff.

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1 Like

Okay, here you are. I hope that helps. Sorry for the ugly code, I'm using R not often enough to remember all the details ;/

Given this data:

``````library(ggplot2)

f <- function(x) { return (1-exp(-x/5)); }

data <- data.frame( x = seq(0, 20) )
data\$y <- sapply(data\$x, f) + rnorm(length(data\$x), sd=0.1)
data

ggplot( data,
aes( x= x,
y= y)) +
theme_minimal()+
geom_point()
``````

I can easily plot normalized to `f(x)` via

``````ggplot( data, aes( x= x, y= y - f(x))) +
theme_minimal()+
geom_point()
``````

and now only quite a few details are missing, that I could try to fix up like this:

``````gridlines0 <- data.frame( x= 0:20, y=0-f(0:20))
gridlines1 <- data.frame( x= 0:20, y=1-f(0:20))
gridlines2 <- data.frame( x= 0:20, y=2-f(0:20))
gridlines05 <- data.frame( x= 0:20, y=0.5-f(0:20))
gridlines05

ggplot( data, aes(x= x, y= y-f(x))) +
scale_y_continuous(breaks = NULL)+
theme_minimal()+
geom_line(data=gridlines0) +
geom_line(data=gridlines1) +
geom_line(data=gridlines2) +
geom_line(data=gridlines05) +
geom_text(data=gridlines0[1,], aes(label="0"), nudge_x=-1) +
geom_text(data=gridlines1[1,], aes(label="1"), nudge_x=-1) +
geom_text(data=gridlines2[1,], aes(label="2"), nudge_x=-1) +
geom_text(data=gridlines05[1,], aes(label="0.5"), nudge_x=-1) +
geom_point()
``````

So, basically I'm transforming the coordinate system, with one axis depending on the location on the other axis (here Y depends on X). Now I'd like a) all the grid, grid labels, etc. to be correct without all that (now manual) stuff, and it would be more ideal if I could give a function of two arguments that returns two arguments (instead of now having `cood_trans` doing two functions of one argument each).

The short answer is “no” - it is not possible to transform coordinate systems in that way in ggplot2. I will not rule out that it is doable to write a Coord class that could perform this, but I think it is much more viable to simply subtract the function value from your data and plot the results. In the end you’d get the same plot but it would probably be easier for people to interpret compared to a plot with some strange coordinate transformation...

2 Likes

Hi Thomas,

well, only subtracting the `f(x)` value isn't enough - the grid doesn't show the absolute values anymore, the scaling isn't optimal (by default), etc. -- I need to manually override quite a lot of things. And if I'd like to transform both X and Y it's even harder.

My point is to enhance the difference between the function value from the model, and the really measured value, but to have the absolute Y values easily available, too.