text() fuction add addtional data sets, which is not expected

[ziyuanbaby]

when add text, and enble the legend, it will show two data sets, the first is as expected, while the second one is trigger by "matplot::text(0, 0, "demo-text")", this is not expected behavoiur.

            using namespace matplot;
            std::vector<double> x = linspace(0, pi);
            std::vector<double> y = transform(x, [&](auto x)
                                              { return cos(x); });
            plot(x, y, "-o");
            matplot::text(0, 0, "demo-text");
            legend();
            show();

as the figure shown, data2 is not expected to be created.