Match Julia histogram behavior

- use Julia-compatible histogram step thresholds
- fix fractional bar glyph rendering
- keep custom histogram xlabel unchanged
- remove unused VHistCanvas
- add histogram reference tests

Author: kojix2

spec/reference_spec.cr

@@ -382,6 +382,59 @@ describe "Julia reference output compatibility" do
       p = UnicodePlot.histogram([0.1_f32, 0.1_f32, 0.0_f32])
       test_ref("histogram/float32.txt", p)
     end
+
+    # The following cases reuse DENSITYPLOT_X which is identical to Julia's
+    # `seed!(RNG, 1337); x = randn(RNG, 4000)` used in tst_histogram.jl.
+
+    it "matches histogram/default" do
+      p = UnicodePlot.histogram(DENSITYPLOT_X)
+      test_ref("histogram/default.txt", p)
+    end
+
+    it "matches histogram/default_1e-2 (data scaled ×0.01)" do
+      p = UnicodePlot.histogram(DENSITYPLOT_X.map { |v| v * 0.01 })
+      test_ref("histogram/default_1e-2.txt", p)
+    end
+
+    it "matches histogram/default_1e2 (data scaled ×100)" do
+      p = UnicodePlot.histogram(DENSITYPLOT_X.map { |v| v * 100.0 })
+      test_ref("histogram/default_1e2.txt", p)
+    end
+
+    it "matches histogram/log10 (xscale: :log10)" do
+      p = UnicodePlot.histogram(DENSITYPLOT_X, xscale: :log10)
+      test_ref("histogram/log10.txt", p)
+    end
+
+    it "matches histogram/log10_label (xscale: :log10 with custom xlabel)" do
+      p = UnicodePlot.histogram(DENSITYPLOT_X, xlabel: "custom label", xscale: :log10)
+      test_ref("histogram/log10_label.txt", p)
+    end
+
+    it "matches histogram/parameters1 (title, xlabel, color:blue, margin, padding)" do
+      p = UnicodePlot.histogram(
+        DENSITYPLOT_X,
+        title: "My Histogram",
+        xlabel: "Absolute Frequency",
+        color: :blue,
+        margin: 7,
+        padding: 3,
+      )
+      test_ref("histogram/parameters1.txt", p)
+    end
+
+    it "matches histogram/parameters2 (title, color:yellow, border:solid, symbols:[=], width:50)" do
+      p = UnicodePlot.histogram(
+        DENSITYPLOT_X,
+        title: "My Histogram",
+        xlabel: "Absolute Frequency",
+        color: :yellow,
+        border: :solid,
+        symbols: ['='],
+        width: 50,
+      )
+      test_ref("histogram/parameters2.txt", p)
+    end
   end
 
   describe "boxplot" do

spec/unicode_plot_spec.cr

@@ -214,9 +214,14 @@ describe UnicodePlot do
       p_right.to_s.should be_a(String)
     end
 
-    it "applies xscale and reflects transformed xlabel" do
+    it "applies xscale; custom xlabel used as-is (no scale suffix), default xlabel gets suffix" do
+      # Custom xlabel: no [log10] suffix (matches Julia behavior)
       p = UnicodePlot.histogram([1.0, 10.0, 100.0], xscale: :log10, xlabel: "x")
-      p.to_s.should contain("x [log10]")
+      p.to_s.should contain("x")
+      p.to_s.should_not contain("x [log10]")
+      # Default xlabel: gets [log10] suffix
+      p2 = UnicodePlot.histogram([1.0, 10.0, 100.0], xscale: :log10)
+      p2.to_s.should contain("Frequency [log10]")
     end
 
     it "accepts RGB tuple color" do

src/unicode_plot/graphics/bar_graphics.cr

@@ -112,8 +112,13 @@ module UnicodePlot
       UnicodePlot.print_color(io, color, @symbols[nsyms - 1].to_s * bar_head, use_color)
       if nsyms > 1
         frac_cells = frac * max_bar_width - bar_head
-        sub_idx = (frac_cells * nsyms).round.to_i32
-        sub_char = sub_idx <= 0 ? ' ' : @symbols[(sub_idx - 1).clamp(0, nsyms - 1)]
+        rem = frac_cells * (nsyms - 2)
+        sub_char = if rem <= 0
+                     ' '
+                   else
+                     ri = rem.round.to_i32
+                     @symbols[ri.clamp(0, nsyms - 2)]
+                   end
         UnicodePlot.print_color(io, color, sub_char.to_s, use_color)
         bar_head += 1
       end

src/unicode_plot/interface/histogram.cr

@@ -4,18 +4,17 @@ module UnicodePlot
     (Math.log2(n) + 1).ceil.to_i32
   end
 
-  # Round step up to next "nice" value ({1,2,2.5,5} × 10^k), matching Julia's histrange.
+  # Round step up to next "nice" value ({1,2,5} × 10^k), matching Julia's StatsBase histrange.
+  # Thresholds 1.1, 2.2, 5.5 are taken directly from StatsBase._histrange_step.
   private def nice_hist_step(step : Float64) : Float64
     return step if step <= 0.0
     mag = 10.0 ** Math.log10(step).floor
     k = step / mag
-    n = if k <= 1.0
+    n = if k <= 1.1
           1.0
-        elsif k <= 2.0
+        elsif k <= 2.2
           2.0
-        elsif k <= 2.5
-          2.5
-        elsif k <= 5.0
+        elsif k <= 5.5
           5.0
         else
           10.0
@@ -293,7 +292,7 @@ module UnicodePlot
       pr2 = " " * (pad_right - (s2.size - dot2))
       "#{l_chr}#{pl1}#{s1}#{pr1}, #{pl2}#{s2}#{pr2}#{r_chr}"
     end
-    barplot(
+    plot = barplot(
       labels_arr, counts,
       color: color, title: title,
       xlabel: xlabel.empty? ? "Frequency" : xlabel,
@@ -303,6 +302,10 @@ module UnicodePlot
       thousands_separator: thousands_separator, symbols: symbols,
       width: width
     )
+    # When a custom xlabel is provided, use it as-is (no scale-name suffix),
+    # matching Julia's behavior where kw... overrides the default transform_name label.
+    plot.xlabel = xlabel unless xlabel.empty?
+    plot
   end
 
   def histogram(data : Array(T), **kwargs) : Plot forall T