fix(stat_summary_bin): give each bin its own width

[Rodrigo-Palma]

With uneven breaks, every summary gets the width of the last bin:

df = pd.DataFrame({"x": range(15), "y": range(15)})
p = ggplot(df, aes("x", "y")) + stat_summary_bin(
    breaks=np.array([0, 2, 10, 14]), fun_y=np.mean, geom="col"
)
p.build().layers[0].data[["xmin", "xmax"]]
# main:  (-1, 3), (4, 8), (10, 14)
# here:  (0, 2), (2, 10), (10, 14)

x already comes from each bin's centre; this takes width per bin the same way. geom_histogram with the same breaks gives (0, 2), (2, 10), (10, 14).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.87%. Comparing base (ac9cfce) to head (5f954fb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1133      +/-   ##
==========================================
- Coverage   88.87%   88.87%   -0.01%     
==========================================
  Files         229      229              
  Lines       17292    17291       -1     
  Branches     2203     2203              
==========================================
- Hits        15368    15367       -1     
  Misses       1295     1295              
  Partials      629      629              

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