{

std::vector<std::vector<double>> x, y, z;

for (double i = -5; i <= 5; i += 0.25) {

std::vector<double> x_row, y_row, z_row;

for (double j = -5; j <= 5; j += 0.25) {

x_row.push_back(i);

y_row.push_back(j);

z_row.push_back(::std::sin(::std::hypot(i, j)));

}

x.push_back(x_row);

y.push_back(y_row);

z.push_back(z_row);

}

plt::contour(x, y, z);

plt::show();