Stacked barplot with colour gradients for each bar

Question

I want to color a stacked barplot so that each bar has its own parent colour, with colours within each bar to be a gradient of this parent colour.

Example:

Answer 1

An easier approach to achieve a colour gradient is to use alpha to change the transparency of the colour. However, this can have unintended consequences as transparency means you can see the guidelines through the plot.

library(ggplot2)

ggplot(diamonds, aes(color, alpha = clarity)) + 
  geom_bar(aes(fill = color), colour = "grey") +
  scale_alpha_discrete(range = c(0,1))