std::vector<double> &xt, std::vector<double> &yt) {

const double target_length = 300;

const double half_win = (target_length / (2. * sqrt(1. + t * t)));

xt[0] = x - half_win;

xt[1] = x + half_win;

yt[0] = y - half_win * t;

yt[1] = y + half_win * t;

size_t n = 1000;

std::vector<double> x, y;

const double w = 0.05;

const double a = n / 2;

for (size_t i = 0; i < n; i++) {

x.push_back(i);

y.push_back(a * sin(w * i));

}

std::vector<double> xt(2), yt(2);

plt::title("Tangent of a sine curve");

plt::xlim(x.front(), x.back());

plt::ylim(-a, a);

plt::axis("equal");

// Plot sin once and for all.

plt::plot(x, y, {{"label", "sin"}});

// Prepare plotting the tangent.

plt::Plot plot("tangent");

plt::legend();

for (size_t i = 0; i < n; i++) {

if (i % 10 == 0) {

update_window(x[i], y[i], a * w * cos(w * x[i]), xt, yt);

// Just update data for this plot.

plot.update(xt, yt);

// Small pause so the viewer has a chance to enjoy the animation.

plt::pause(0.1);

}

}