r - How can I pass individual `curvature` arguments in `ggplot2` `geom_curve` function?

Question

I have a df with two curve definitions, each consists of two points and a curvature value. The goal is to plot two individual curves using ggplot2 geom_curve (or an alternative).

I can generate my expected output using:

df <- data.frame(x = c(0,.2), y = c(0,.3), xend = c(1,.4), yend = c(1,.6), curvature = c(-.2,.4))
ggplot(df) + geom_curve(data = df[1, ], aes(x = x, y = y, xend = xend, yend = yend), curvature = df$curvature[1]) + geom_curve(data = df[2, ], aes(x = x, y = y, xend = xend, yend = yend), curvature = df$curvature[2])

But this is not really a solution, since in my real case I have lot more curves (and I don't know how many in advance).

How can I pass an individual curvature argument to the geom_curve call?

---

I tried:

df <- data.frame(x = c(0,0), y = c(0,0), xend = c(1,1), yend = c(1,1), curvature = c(-.2,.8))
library(ggplot2)
ggplot(df) + geom_curve(aes(x = x, y = y, xend = xend, yend = yend, curvature = curvature))

This plots both curves on top of each other and throws an additional warning:

Warning: Ignoring unknown aesthetics: curvature

---

So I tried:

ggplot(df) + geom_curve(aes(x = x, y = y, xend = xend, yend = yend), curvature = curvature)

This throws an error:

Error in layer(data = data, mapping = mapping, stat = stat, geom = GeomCurve, : object 'curvature' not found

---

So I tried to explicitly pass the curvature colon:

ggplot(df) + geom_curve(aes(x = x, y = y, xend = xend, yend = yend), curvature = df$curvature)

This as well throws an error:

Error in seq.default(0, dir * maxtheta, dir * maxtheta/(ncp + 1)) :
'to' muss L?nge 1 haben In addition: Warning messages: 1: In if (curvature == 0) { : the condition has length > 1 and only the first element will be used 2: In if (curvature > 0) hand <- "right" else hand <- "left" : the condition has length > 1 and only the first element will be used

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From @markus' solution I learned, that we can pass lists to a ggplot object, so I tried:

ggplot(df) + 
  lapply(df$curvature, function(i) {
    geom_curve(aes(x = x, y = y, xend = xend, yend = yend), curvature = i) }
  )

But this plots each curve with both curvature argument:

---

How can I pass that curvature argument individually for each row?

Answer 1

update

You might split your data first and then use lapply to iterate over the resulting list which we'll feed to the data argument of geom_curve()

df2 <- data.frame(x = c(0,.2), y = c(0,.3), xend = c(1,.4), yend = c(1,.6), curvature = c(-.2,.4))
ggplot() + 
  lapply(split(df2, 1:nrow(df)), function(dat) {
    geom_curve(data = dat, aes(x = x, y = y, xend = xend, yend = yend), curvature = dat["curvature"]) }
  )

original ansewr

curvature is not an aesthetic, as you have noted. You can add a list to ggplot(), to get it work

df <- data.frame(x = c(0,0), y = c(0,0), xend = c(1,1), yend = c(1,1), curvature = c(-.2,.8))
ggplot(df) + 
  lapply(df$curvature, function(i) {
    geom_curve(aes(x = x, y = y, xend = xend, yend = yend), curvature = i) }
    )

From help("+.gg")

What can you add?

...

You can also supply a list, in which case each element of the list will be added in turn.

---

If you have other parameters that you want to show in your plot - each line might be coloured differently, is of different size etc. - use Map

Modified data

df1 <- data.frame(x = c(0,0), y = c(0,0), xend = c(1,1), yend = c(1,1), curvature = c(-.2,.8),
                  colour = c("red", "blue"))

Plot

ggplot(df1) + 
  Map(function(i, col) {
    geom_curve(aes(x = x, y = y, xend = xend, yend = yend), curvature = i, colour = col) },
    i = df1$curvature, col = df1$colour
  )

Result