File:Slope Field-modified.svg

library(ggplot2)

f <- function(x, beta0) {
  (1 / 3) * x ^ 3 - (1 / 2) * x ^ 2 - 2 * x + beta0
}

d <-
  expand.grid(x = seq(-4, 4, 0.01), Intercept = c(-4, 0, 4))
d$y <- with(d, f(x, Intercept))

d.slope <-
  expand.grid(x = seq(-4, 4, 0.5),
              y = seq(-10, 10, 0.5))
d.slope$slope <- with(d.slope, x ^ 2 - x - 2)
d.slope$angle <- with(d.slope, atan(slope) / pi * 180)

ggplot(d, aes(x, y)) +
  geom_hline(yintercept = 0) +
  geom_vline(xintercept = 0) +
  geom_text(data = d.slope,
            aes(angle = angle, label = "—"),
            color = 3,
            size = 2.5) +
  geom_line(aes(color = as.factor(Intercept), group = as.factor(Intercept), size = as.factor(Intercept))) +
  scale_x_continuous("x", breaks = seq(-5, 5, 1)) +
  scale_y_continuous("y", breaks = seq(-50, 50, 1), limits = c(-8, 6)) +
  theme(panel.background = element_blank()) +
  theme(text = element_text(size = 12)) +
  theme(legend.position = "top") +
  scale_color_manual(values = c('green', 'red', 'blue')) +
  scale_size_manual(values = 1:3) +
  guides(size = guide_legend("Intercept"), color = guide_legend("Intercept")) +
  coord_fixed() +
  windows(5, 6)