Ggplot() in R - Rescaling Primary and Secondary Axes

I have some code that will draw a nice "combo" graph with separate primary and secondary axes (which is what I need for a particular application). A simple example here:

library(tidyverse)

df <- data.frame(star_sign = c("Aries", "Taurus", "Gemini", "Cancer", "Leo", "Virgo", "Libra", "Scorpio", "Sagittarius", "Capricorn", "Aquarius", "Pisces"),
                 weight = c(50, 70, 60, 40, 45, 78, 42, 22, 28, 49, 50, 31),
                 mean = c(1.1, 1.2, 1.4, 1.3, 1.8, 1.6, 1.4, 1.3, 1.2, 1.1, 1.5, 1.3))

lvls = c("Aries", "Taurus", "Gemini", "Cancer", "Leo", "Virgo", "Libra", "Scorpio", "Sagittarius", "Capricorn", "Aquarius", "Pisces")

a <- 0
b <- 0.01
primary_y_min <- -1
primary_y_max <- 2

df %>% 
  mutate(star_sign = as.numeric(factor(star_sign, levels = lvls))) %>% 
  ggplot(aes(star_sign)) + 
  geom_line(aes(y = mean), color = "red") + 
  geom_col(aes(y = (weight + a) * b), width = 0.5, fill = "blue") + 
  scale_y_continuous("values", limits = c(primary_y_min, primary_y_max), sec.axis = sec_axis(~ ./b - a, name = "weight")) +  
  scale_x_continuous("X title", breaks = c(1:12), labels = lvls) 

Tweaking the values of a, b, primary_y_min, and primary_y_max gives me some control over the axes (the primary axis in particular). But it's not quite giving me what I need. I'd like to be able to choose:

• Primary axis: min and max y

• Secondary axis: max y (min y will always be 0)

Is there a way to do this? I would hope it was fairly easy (!) But my knowledge of ggplot2 is minimal so I'm not sure how to go about this.