Mapping -diff(y) to linewidth = "light on the upstroke, heavy on the downstroke" = calligraphy
```
library(tidyverse)
library(pracma)
tibble(s = seq(pi, -pi, len = 5000),
x = fresnelS(s), y = fresnelC(s),
wd = c(0, -diff(y))) |>
ggplot() +
geom_path(aes(x=x,y=y, lwd=wd), show.legend=FALSE) +
scale_linewidth_continuous(range=c(.1,2.5)) +
theme_void()
```