Geom.bar update 1

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• adds Geom.bar(position=:identity). identity means not stacked or dodged
• enables alpha aesthetic for position=:identity
• fixes Histogram with alpha values, bars overlapping or bar highlight not same as bar color? #810: overlapping histograms with same bins (see plot p3 below)
• and maybe How can I change the colors of bars of a bar plot? (No grouping) #1422 (alpha can be used to shade or noise bar color)

Example

using DataFrames, Distributions
x = range(-4, 4, length=30)
fn1(μ,x=x) = pdf(Normal(μ, 1), x)
D = [DataFrame(x=x, y=fn1(μ), μ="$(μ)") for μ in [-1, 1]]
p1 = plot(D[1], x=:x, y=:y, Geom.bar, alpha=range(0.2,0.9, length=30))
p2 = plot(vcat(D...), x=:x, y=:y, color=:μ, alpha=[0.5], Geom.bar(position=:identity))
a = repeat([0.75, 0.85], outer=40) # opacity
D2 = [DataFrame(x=rand(Normal(μ,1), 500), μ="$(μ)") for μ in [-1, 1]]
p3 = plot(vcat(D2...), x=:x,  color=:μ, alpha=[a;a],
 Geom.histogram(position=:identity, bincount=40, limits=(min=-4, max=4)),
    Scale.color_discrete_manual("skyblue","moccasin"))
hstack(p1, p2, p3)

[codecov-io]

Codecov Report

Merging #1428 into master will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #1428      +/-   ##
==========================================
+ Coverage   89.42%   89.43%   +0.01%     
==========================================
  Files          39       39              
  Lines        4320     4325       +5     
==========================================
+ Hits         3863     3868       +5     
  Misses        457      457              

Impacted Files	Coverage Δ
src/geom/bar.jl	93.57% <87.50%> (+0.23%)	⬆️

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