Add support for nxxm.

[nxxm](https://nxxm.github.io)

src/ftxui/dom/graph.cpp

@@ -0,0 +1,45 @@
+#include "ftxui/dom/elements.hpp"
+
+namespace ftxui {
+
+const wchar_t charset[] = L" ▗▐▖▄▟▌▙█";
+
+class Graph : public Node {
+ public:
+  Graph(GraphFunction graph_function) : graph_function_(graph_function) {}
+  ~Graph() override {}
+
+  void ComputeRequirement() override {
+    requirement_.flex.x = 1;
+    requirement_.flex.y = 1;
+    requirement_.min.x = 1;
+    requirement_.min.y = 1;
+  }
+
+  void Render(Screen& screen) override {
+    int width = (box_.x_max - box_.x_min + 1) * 2;
+    int height = (box_.y_max - box_.y_min + 1) * 2;
+    auto data = graph_function_(width, height);
+    int i = 0;
+    for (int x = box_.x_min; x <= box_.x_max; ++x) {
+      int height_1 = 2 * box_.y_max - data[i++];
+      int height_2 = 2 * box_.y_max - data[i++];
+      for (int y = box_.y_min; y <= box_.y_max; ++y) {
+        int yy = 2 * y;
+        int i_1 = yy < height_1 ? 0 : yy == height_1 ? 3 : 6;
+        int i_2 = yy < height_2 ? 0 : yy == height_2 ? 1 : 2;
+        wchar_t pix = charset[i_1 + i_2];
+        screen.at(x, y) = pix;
+      }
+    }
+  }
+
+ private:
+  GraphFunction graph_function_;
+};
+
+std::unique_ptr<Node> graph(GraphFunction graph_function) {
+  return std::make_unique<Graph>(graph_function);
+}
+
+};  // namespace ftxui