bug: geom_text in middle of stacked bar_plot not in the middle (v2.1.0.9001)

[dpprdan]

The placement of geom_text in the middle of a stacked bar_plot is off. When I run the example from the help file of geom_text I get:

df <- data.frame(
  x = factor(c(1, 1, 2, 2)),
  y = c(1, 3, 2, 1),
  grp = c("a", "b", "a", "b")
)

df <- transform(df, mid_y = ave(df$y, df$x, FUN = function(val) cumsum(val) - (0.5 * val)))

ggplot(data = df, aes(x, y, fill = grp, label = y)) +
  geom_col() +
  geom_text(aes(y = mid_y))

I suppose this is due to "position_stack() and position_fill() now stack values in the reverse order of the grouping, which makes the default stack order match the legend."

[aosmith16]

The dataset will need to be arranged in the reverse order of the fill factor to calculate the positions correctly with cumsum.

df <- transform(df[order(rev(df$grp)), ], mid_y = ave(y, x, FUN = function(val) cumsum(val) - (0.5 * val)))

[dpprdan]

That's a possibility, of course, but a bad one at that, IMHO. Because reversing the order of the grouping of the bars under the hood without reversing the order of the labels is a breaking change. After all the example from the geom_text help file is working in v2.1 and will break, as it stands, in v2.2, and so will all similar bar charts created with ggplot2 v2.1 and before. So is there a chance to adapt geom_text to the new grouping order?

[hadley]

One possible resolution would be a new position adjustment that "stacks" text like this, including an optional parameter (0-1) that lets you control where the text appears within the rectangular region. That wouldn't fix existing code the relies on the specific stacking order, but it would provide a simple fix.

[hadley]

Here's a quick implementation:

#' @example
#' df <- data.frame(
#' x = factor(c(1, 1, 2, 2)),
#' y = c(1, 3, 2, 1),
#' grp = c("a", "b", "a", "b")
#' )
#'
#' ggplot(data = df, aes(x, y, fill = grp, label = y)) +
#'   geom_col() +
#'   geom_text(position = "stack_point")
position_stack_point <- function(vjust = 0.5) {
  ggproto(NULL, PositionStackPoint,
    vjust = vjust
  )
}

PositionStackPoint <- ggproto("PositionStackPoint", Position,
  required_aes = c("x", "y"),
  vjust = 0.5,

  setup_params = function(self, data) {
    list(vjust = self$vjust)
  },

  compute_layer = function(self, data, params, panel) {
    data <- data[order(data$x, -data$group), ]
    data$y <- ave(data$y, data$x, FUN = function(x) cumsum(x) - ((1 - params$vjust) * x))
    data
  }
)

[hadley]

I now think that this should be integrated with position_stack so it does the right thing for both points and bars

[thomasp85]

We could have a just argument that could control that. It works fine for points as it is now (I.e. It stacks them) but this would give users even more control

[hadley]

That's what I'm in the middle of implementing 😄