DarkThemeNeonIRTplot

#libs--------------
library(grid)
library(tidyverse)
library(ggdark)
#funs--------------

irt <- function(a,b,c){
  theta <- seq(-10,10, length = 500)
  exponent <- exp(a*(theta-b))
  phat <- c + (1-c)*(exponent/(1 + exponent))
  return(data.frame(p = phat, theta = theta,
                    a = a, b = b, c = c))
}


#difficulty----
diff_df <- lapply(seq(-5,5,by = 1), function(x) irt(1,x,0))
diff_df <- do.call(rbind, diff_df)

ggplot(diff_df, aes(x = theta, y = p, color = factor(b))) +
  geom_line(size = 1) +
  dark_theme_grey() +
  scale_y_continuous(limits = c(0,1),
                     breaks = c(0,0.5,1)) + 
  scale_x_continuous(limits = c(-10,10)) + 
  scale_color_discrete(name = "Difficulty") +
  theme(panel.grid.minor = element_blank(),
        panel.grid.major.x = element_blank(),
        axis.title = element_text(size = 20),
        axis.text = element_text(size = 15),
        plot.title = element_text(size = 25)) +
  labs(x = "Latent Trait Value",
       y = "Predicted Probability",
       title = "Logistic Curves for Items\nWith Different Difficulties")