position_stack() should stack +ive & -ve values separately

[hadley]

library(ggplot2)
library(tibble)

df <- frame_data(
  ~x, ~g, ~y,
  1,  1,  1,
  1,  2,  -1,
  1,  3,  1,
  2,  1,  2,
  2,  2,  -3
)

ggplot(df, aes(x, y, fill= factor(g))) + 
  geom_bar(stat = "identity")

[thomasp85]

There are a lot of case where, even if supported, negative values doesn't make sense when stacking. As I see it it only makes sense for stacked bar charts - can you come with other examples...

I'm asking because I need to know which types of checks to make and warnings to throw, and also where it should be solved. It could be handled directly by pos_stack() but this function is also called by pos_fill() where it doesn't make sense to have negative values. Alternatively (and easier) the data could be split into two calls to collide in PositionStack$compute_panel() which would make the functionality specific to PositionStack (though it would still be a mess if coupled with geom_area

[hadley]

Can you come with an example where stacking makes sense apart from bars/areas? I think that's the only case we need to worry about, and it's reasonable for positive values to stack upwards from the y-axis, and negative values to stack downwards.

Your fix seems reasonable to me - I don't think there's any way to have an elegant solution without completely rewriting the stacking (which we don't want to do)

[thomasp85]

I cannot - which was why I asked whether you knew of any other use cases. I'll move forward with #1693

[smouksassi]

I think we can also stack dots which can be useful in some cases. http://www.lispworks.com/success-stories/inspiredata.html

http://docs.ggplot2.org/current/geom_dotplot.html

[thomasp85]

@smouksassi true. Dot plots should mainly be used for count data though, which makes them unsuitable to show negative values.

[smouksassi]

I was thinking about a kind of back to back dotplot like this: https://nzmaths.co.nz/resource/fully-stretched:

[hadley]

Dot plots have to use a completely different algorithm so aren't relevant to this discussion.

[wmurphyrd]

Here's an example where the current behavior of position_stack for a mixture of positive and negative values produced the desired result: the total extent of the bar aligns with the sum of the values (please ignore the politics, just examples that I happened to have handy).
. However, I did zoom-hide the negative side because it's not clear what's going on. (From https://github.com/wmurphyrd/BernieTax )

In another example, I wanted to show what was happening on the negative side with a dodge:
. Although, in the end I used geom_tile with position_dodge after calculating the offsets manually