Function transform

source/matplot/util/common.cpp:102–109

    std::vector<double> iota(double d1, double d2) { return iota(d1, 1., d2); }

    std::vector<double> transform(const std::vector<double> &x,
                                  std::function<double(double)> fn) {
        std::vector<double> y(x.size());
        for (size_t i = 0; i < y.size(); ++i) {
            y[i] = fn(x[i]);
        }
        return y;
    }

    double truncate(double x, double lower_bound, double upper_bound) {
        if (x <= lower_bound) {