added example for fill_inbetween.cpp

examples/fill_inbetween.cpp

@@ -0,0 +1,28 @@
+#define _USE_MATH_DEFINES
+#include "../matplotlibcpp.h"
+#include <cmath>
+#include <iostream>
+
+using namespace std;
+namespace plt = matplotlibcpp;
+
+int main() {
+  // Prepare data.
+  int n = 5000;
+  std::vector<double> x(n), y(n), z(n), w(n, 2);
+  for (int i = 0; i < n; ++i) {
+    x.at(i) = i * i;
+    y.at(i) = sin(2 * M_PI * i / 360.0);
+    z.at(i) = log(i);
+  }
+
+  // Prepare keywords to pass to PolyCollection. See
+  // https://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.fill_between.html
+  std::map<string, string> keywords;
+  keywords["alpha"] = "0.4";
+  keywords["color"] = "grey";
+  keywords["hatch"] = "-";
+
+  plt::fill_between(x, y, z, keywords);
+  plt::show();
+}