Introduction
Creating a symlog scale in ggplot or lattice, similar to Matplotlib’s symlog scale, can be challenging due to the complex nature of tick mark and label placement. However, with the use of the scales package in R, it is possible to achieve this behavior.
In this article, we will explore how to create a symlog scale in ggplot using the scales package. We will also discuss the differences between the Python version of the symlog scale and the R implementation.
Understanding Symlog Scales
A symlog scale is a type of logarithmic scale that uses a linear transformation outside some interval, and a logarithmic transformation inside it. This allows for the representation of both positive and negative values on the same axis.
The Python version of the symlog scale uses the following formula to transform x:
thr + scale * suppressWarnings(log(sign(x) * x / thr, base))
This formula takes the absolute value of x, subtracts a threshold value (thr), logs the result, and then adds the scaled logarithm.
Implementing Symlog Scale in R using Scales Package
The scales package provides a function called symlog_trans that can be used to create a symlog scale.
symlog_trans <- function(base = 10, thr = 1, scale = 1){
trans <- function(x)
ifelse(abs(x) < thr, x, sign(x) *
(thr + scale * suppressWarnings(log(sign(x) * x / thr, base))))
inv <- function(x)
ifelse(abs(x) < thr, x, sign(x) *
base^((sign(x) * x - thr) / scale) * thr)
breaks <- function(x){
sgn <- sign(x[which.max(abs(x))])
if(all(abs(x) < thr))
pretty_breaks()(x)
else if(prod(x) >= 0){
if(min(abs(x)) < thr)
sgn * unique(c(pretty_breaks()(c(min(abs(x)), thr)),
log_breaks(base)(c(max(abs(x)), thr))))
else
sgn * log_breaks(base)(sgn * x)
} else {
if(min(abs(x)) < thr)
unique(c(sgn * log_breaks()(c(max(abs(x)), thr)),
pretty_breaks()(c(sgn * thr, x[which.min(abs(x))]))))
else
unique(c(-log_breaks(base)(c(thr, -x[1])),
pretty_breaks()(c(-thr, thr)),
log_breaks(base)(c(thr, x[2]))))
}
}
trans_new(paste("symlog", thr, base, scale, sep = "-"), trans, inv, breaks)
}
This function takes the following parameters:
base: The base of the logarithm (default is 10).thr: The threshold value (default is 1).scale: The scaling factor for the logarithmic transformation.
Example Usage
To create a symlog scale in ggplot, you can use the following code:
data <- data.frame(x = seq(-50, 50, 0.01), y = seq(0, 100, 0.01))
data$y2 <- sin(data$x / 3)
ggplot(data, aes(x, y)) + geom_line() + theme_bw() +
scale_x_continuous(trans = symlog_trans())
This will create a plot with a symlog x-axis and a linear y-axis.
Conclusion
In this article, we have explored how to create a symlog scale in ggplot using the scales package. We have also discussed the differences between the Python version of the symlog scale and the R implementation. With the use of the scales package, it is now possible to achieve this complex scaling behavior in R.
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Last modified on 2024-04-07