// Prepare data.

vector<double> theta;

for (double d = 0; d < 8 * M_PI; d += 0.1)

theta.push_back(d);

const int a = 1;

const double b = 0.2;

for (double dt = 0; dt < 2 * M_PI; dt += M_PI/2.0) {

vector<double> x1, y1, x2, y2;

for (double th : theta) {

x1.push_back( a*cos(th + dt) * exp(b*th) );

y1.push_back( a*sin(th + dt) * exp(b*th) );

x2.push_back( a*cos(th + dt + M_PI/4.0) * exp(b*th) );

y2.push_back( a*sin(th + dt + M_PI/4.0) * exp(b*th) );

}

x1.insert(x1.end(), x2.rbegin(), x2.rend());

y1.insert(y1.end(), y2.rbegin(), y2.rend());

plt::fill(x1, y1, {});

}

plt::show();