multi line plot dose not work as expected

[ziyuanbaby]

           using namespace matplot;
            hold(on);
            for (int i = 0; i < 5; i++)
            {
                std::vector<double> x = linspace(0, pi);
                std::vector<double> y1 = transform(x, [&](auto x)
                                                   { return cos(x * (i + 1)); });
                plot(x, y1, "-o");
            }
          
            legend();
            show();

the above code draw 5 lines. but only the first one is shown on the figure.

however, when i only draw three lines, it works as expected. can anyone tell me where i am doing wrong?

            using namespace matplot;
            hold(on);
            for (int i = 0; i < 3; i++)
            {
                std::vector<double> x = linspace(0, pi);
                std::vector<double> y1 = transform(x, [&](auto x)
                                                   { return cos(x * (i + 1)); });
                plot(x, y1, "-o");
            }

            legend();
            show();