shadow_mark.md

shadow_mark

Danielle Navarro & Dale Maschette 22/11/2018

This shadow_mark() walk through extends the shadow_wake() walk through and uses the same animation.

ntimes <- 20  # how many time points to run the bridge?
nseries <- 5 # how many time series to generate?

# function to generate the brownian bridges
make_bridges <- function(ntimes, nseries) {
  replicate(nseries, c(0,rbridge(frequency = ntimes-1))) %>% as.vector()
}

# construct tibble
tbl <- tibble(
  Time = rep(1:ntimes, nseries),
  Horizontal = make_bridges(ntimes, nseries),
  Vertical = make_bridges(ntimes, nseries),
  Series = gl(nseries, ntimes)
)

# construct the base picture
base_pic <- tbl %>%
  ggplot(aes(
    x = Horizontal, 
    y = Vertical, 
    colour = Series)) + 
  geom_point(
    show.legend = FALSE,
    size = 5) + 
  coord_equal() + 
  xlim(-1.5, 1.5) + 
  ylim(-1.5, 1.5)

# base animation with no shadow
base_anim <- base_pic + transition_time(time = Time) 
base_anim %>% animate()

See the other walk through for details.

Minimal use of shadow mark

mark0 <- base_anim + shadow_mark()
mark0 %>% animate(type = "cairo")

Setting fixed aesthetics

mark1<- base_anim + 
  shadow_mark(
    size = 2, 
    alpha = 0.8,
    colour = "black"
  )
mark1 %>% animate(type = "cairo")

Excluding layers

Suppose we want to solve Dale's problem in a much simpler way: keep the colours on the shadow mark, but have the original dots all be black. Solution is to add another layer but exclude it from the shadow mark:

new_pic <- base_pic + 
  geom_point(colour = "black", size = 5)

mark2 <- new_pic + 
  transition_time(time = Time) + 
  shadow_mark(size = 2, exclude_layer = 2)

mark2 %>% animate(type = "cairo")

In some cases the shadow mark might need to rendered using a different colour palette than the main points. This is slightly trickier as ggplot2 isn't really designed to allow different layers to use different palettes. We can work around it using scale_colour_manual() like so:

# add a dummy variable Series2
tbl <- tbl %>% 
  mutate(Series2 = as.factor(as.numeric(Series) + nseries))

# Create a colour map that attaches a colour to each 
# named value in Series or Series2. For simplicity they're both
# rainbow() maps, one highly desaturated and the other one not
cmap <- c(rainbow(nseries, s = .4), rainbow(nseries, s = .8))
names(cmap) <- 1:(2*nseries)

# Vonstruct the plot with two geom_point layers
twin_pic <- tbl %>%
  ggplot(aes(
    x = Horizontal, 
    y = Vertical, 
    colour = Series)) + 
  geom_point( # layer 1
    show.legend = FALSE,
    size = 5) +
  geom_point( # layer 2
    mapping = aes(colour = Series2),
    show.legend = FALSE,
    size = 5    
  ) +
  scale_color_manual(values = cmap) +
  coord_equal() + 
  xlim(-1.5, 1.5) + 
  ylim(-1.5, 1.5)

# Animation with shadow mark, excluding layer 2
mark3 <- twin_pic + 
  transition_time(time = Time) +
  shadow_mark(size = 2, exclude_layer = 2)

mark3 %>% animate(type = "cairo")

Shadow marks from the future

By default shadow_mark() displays marks from earlier states but not future state, but this is flexible. The default parameters past = TRUE and future = FALSE can both be modified. For example, to show the shadow mark only for future points:

mark4 <- base_anim + 
  shadow_mark(
    past = FALSE,
    future = TRUE,
    colour = "grey70"
  )
mark4 %>% animate(type = "cairo")