feat(plot): Figure with stacked rows and a synchronised x axis

tests/CMakeLists.txt

@@ -35,6 +35,7 @@ add_executable(plot_tests
   test_ticks.cpp
   test_measure.cpp
   test_plot.cpp
+  test_figure.cpp
   test_render_smoke.cpp
   test_controller.cpp
   egl_headless.cpp
@@ -134,9 +135,11 @@ file(GENERATE OUTPUT "${plot_umbrella_check_source}" CONTENT
 
 int main() {
   plot::Plot plot;
+  plot::Figure figure(2);
   plot::Renderer renderer;
   plot::data::StreamSeries series(16, 0.0, 1.0);
   plot::input::Controller controller(plot);
+  (void)figure;
   (void)renderer;
   (void)series;
   (void)controller;