examples/update.cpp

#define _USE_MATH_DEFINES
#include <cmath>
#include "../matplotlibcpp.h"
#include <chrono>

namespace plt = matplotlibcpp;

void update_window(const double x, const double y, const double t,
                   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;
}


int main()
{
    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::named_plot("sin", x, y);

    // 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);
        }
   }
}