Allow stacked bars for data containing negative y-values in `type_barplot()`

[634523]

When plotting a stacked bar chart with negative values, tinyplot doesn't stack them the way ggplot2 does for example. Currently for data that contains negative y-values the following warning is shown:

Warning message:
In settings$type_data(settings, ...) :
  'beside' must be TRUE if there are negative 'y' values

and the bars are put beside. For some data it would make sense to show the bars stacked even if some values are negative (e.g. balances like below):

rm(list = ls())

library(tinyplot)
library(worldbank)
library(ggplot2)

df <- wb_data(
  indicator = c("BN.GSR.GNFS.CD", "BN.GSR.FCTY.CD", "BN.TRF.CURR.CD"),
  country = "US"
)

# tinyplot
plt(
  value ~ date | indicator_id,
  data = df,
  type = "barplot"
)

# ggplot2
ggplot(df, aes(fill=indicator_id, y=value, x=date)) + 
  geom_bar(position="stack", stat="identity")

[zeileis]

Hmm, if the signs of the value were always the same within each indicator_id group, then I can see the value of this plot. If the signs change (as in your example), then I find the plot somewhat hard to interpret. This is brought out more clearly when we focus on the first and on the last years:

I think it would be reasonably easy to implement the stacked height as the absolute sum of all heights and then center by the sum of the negative heights. But the reordering of the categories is not completely obvious in the code. Let's see what Grant @grantmcdermott says when he's back from vacation.