Scale relative to a value in each group (via dplyr)

Question

I have a set of time series, and I want to scale each of them relative to their value in a specific interval. That way, each series will be at 1.0 at that time and change pr

Answer 1

Use first in dplyr, ensuring you use order_by

data %>% 
  group_by(category) %>% 
  mutate(value = value / first(value, order_by = year))