std::chi_squared_distribution

#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <random>
#include <vector>

template <int Height = 5, int BarWidth = 1, int Padding = 1, int Offset = 0,
          bool DrawMinMax = true, class Sample>
void draw_vbars(Sample const& s) {
    static_assert((Height > 0) && (BarWidth > 0) && (Padding >= 0) && (Offset >= 0));
    auto cout_n = [](auto const& v, int n) { while (n-- > 0) std::cout << v; };
    const auto [min, max] = std::minmax_element(std::cbegin(s), std::cend(s));
    std::vector<std::div_t> qr;
    for (float e : s) {
        qr.push_back(std::div(std::lerp(0.f, Height*8, (e - *min)/(*max - *min)), 8));
    }
    for (auto h{Height}; h-- > 0 ;) {
        cout_n(' ', Offset);
        for (auto [q, r] : qr) {
            char d[] = ""; // == { 0xe2, 0x96, 0x88, 0 }
            q < h ? d[0] = ' ', d[1] = '\0' : q == h ? d[2] -= (7 - r) : 0;
            cout_n(d, BarWidth);
            cout_n(' ', Padding);
        }
        if (DrawMinMax && Height > 1)
            h == Height - 1 ? std::cout << " " << *max:
                     h != 0 ? std::cout << ""
                            : std::cout << " " << *min;
        cout_n('\n', 1);
    }
}

int main() {
    std::random_device rd{};
    std::mt19937 gen{rd()};

    auto 2 = [&gen](const float dof) {
        std::chi_squared_distribution<float> d{ dof /* n */ };

        const int norm = 1'00'00;
        const float cutoff = 0.002f;

        std::map<int, int> hist{};
        for (int n=0; n!=norm; ++n) { ++hist[std::round(d(gen))]; }

        std::vector<float> bars;
        std::vector<int> indices;
        for (const auto [n, p] : hist) {
            if (float x = p * (1.0/norm); cutoff < x) {
                bars.push_back(x);
                indices.push_back(n);
            }
        }

        std::cout << "dof = " << dof << ":\n";
        draw_vbars<4,3>(bars);
        for (int n : indices) { std::cout << "" << std::setw(2) << n << "  "; }
        std::cout << "\n\n";
    };

    for (float dof : {1.f, 2.f, 3.f, 4.f, 6.f, 9.f}) 2(dof);
}