code: сделан индивидуальный вариант

histogram_internal.h

@@ -1,8 +1,11 @@
 #ifndef HISTOGRAM_INTERNAL_H_INCLUDED
 #define HISTOGRAM_INTERNAL_H_INCLUDED
 #include <vector>
+#include <string>
 
 void
 find_minmax(const std::vector<double>& numbers, double& min, double& max);
 
+std::string
+bin_color(size_t bin, size_t max_count);
 #endif // HISTOGRAM_INTERNAL_H_INCLUDED

svg.cpp

@@ -28,6 +28,10 @@ svg_rect(double x, double y, double width, double height, string stroke, string
     cout << "<rect x = '" << x << "' y = '" << y << "' width = '" << width << "' height = '" << height << "' stroke = '" << stroke << "' fill = '" << fil << "' />";
 }
 
+string bin_color(size_t bin, size_t max_count){
+    return ("#" + to_string(10 - (bin * 9) / max_count) + to_string(10 - (bin * 9) / max_count) + to_string(10 - (bin * 9) / max_count));
+}
+
 void
 show_histogram_svg(const vector<size_t>& bins) {
     const auto IMAGE_WIDTH = 400;
@@ -46,12 +50,12 @@ show_histogram_svg(const vector<size_t>& bins) {
         }
     }
 
-
     double top = 0;
     for (size_t bin : bins) {
         const double bin_width = (IMAGE_WIDTH - TEXT_WIDTH) * bin / max_count;
+        const auto color = bin_color(bin,max_count);
         svg_text(TEXT_LEFT, top + TEXT_BASELINE, to_string(bin));
-        svg_rect(TEXT_WIDTH, top, bin_width, BIN_HEIGHT,"red","#ffeeee");
+        svg_rect(TEXT_WIDTH, top, bin_width, BIN_HEIGHT,"grey", color);
         top += BIN_HEIGHT;
     }
     svg_end();

svg.h

@@ -15,6 +15,9 @@ show_histogram_svg(const std::vector<size_t>& bins);
 void
 svg_text(double left, double baseline, std::string text);
 
+std::string
+bin_color(size_t bin, size_t max_count);
+
 void
 svg_rect(double x, double y, double width, double height, std::string stroke = "black", std::string fil = "black");
 

unittest.cpp

@@ -3,6 +3,8 @@
 #include "doctest.h"
 #include "histogram_internal.h"
 
+using namespace std;
+
 TEST_CASE("distinct positive numbers") {
     double min = 0;
     double max = 0;
@@ -10,3 +12,13 @@ TEST_CASE("distinct positive numbers") {
     CHECK(min == 1);
     CHECK(max == 2);
 }
+
+TEST_CASE("bin color") {
+    size_t max_bin = 10;
+    size_t min_bin = 3;
+    string rat = "#" + to_string(10 - (min_bin * 9) / max_bin) + to_string(10 - (min_bin * 9) / max_bin) + to_string(10 - (min_bin * 9) / max_bin);
+    auto test1 = bin_color(max_bin,max_bin);
+    auto test2 = bin_color(min_bin,max_bin);
+    CHECK(test1 == "#111");
+    CHECK(test2 == rat);
+}