scale_alpha

Visually down-weight less important observations and then convert them with ggplotly

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(alpha = year))

plotly::ggplotly(p)

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(alpha = year))
p <-  p + scale_alpha("cylinders")

plotly::ggplotly(p)

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(alpha = year))
p <-  p + scale_alpha(range = c(0.4, 0.8))

plotly::ggplotly(p)

GGPLOT - scale_alpha