fix two-sided p-value shading

[simonpcouch]

Fixes #424.

The geom_area height of p-value shading was twice the height of the respective histogram bars when the calculated statistic fell exactly on the boundaries of the right-most boundary of the left shaded region and left-most boundary of the right shaded region.🤯

This PR correct the p-value shading in those cases:

library(infer)

r_hat <- gss %>% 
  observe(college ~ sex, success = "no degree",
          stat = "ratio of props", order = c("female", "male"))

set.seed(33)

null_dist <- gss %>%
  specify(college ~ sex, success = "no degree") %>%
  hypothesize(null = "independence") %>% 
  generate(reps = 1000, type = "permute") %>% 
  calculate(stat = "ratio of props", order = c("female", "male"))


visualize(null_dist) +
  shade_p_value(obs_stat = r_hat, direction = "two-sided")

^{Created on 2021-12-30 by the reprex package (v2.0.0)}

The fix nudges the right-hand boundary of the right-most area geom in the left-hand shading region 1/100000-th of the plot scale to the left. This should be visually indistinguishable, but will cause all of the (already failing) vdiffr tests to newly fail. Will come back to this once #428 is fixed.

[mine-cetinkaya-rundel]

I like the nudging solution, seems simple enough and accomplishes the task.

[andrewpbray]

Phew, good find and a practical solution.

[github-actions]

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