code: public static void main(String[] argv) throws IOException, InterruptedException {
      final File gnuPlotBin = new File(argv[0]);
      final List<ImmutableList<Double>> data = ImmutableList.of(
          ImmutableList.of(0.0, 5.0, 2.0, 0.0, 1.0),
          ImmutableList.of(1.0, 4.0, 3.0, 0.0, 2.0),
          ImmutableList.of(5.0, 2.0, 0.0, 1.0, 1.0),
          ImmutableList.of(9.0, 4.0, 1.0, 3.0, 0.0),
          ImmutableList.of(14.0, 6.0, 2.0, 0.0, 3.0));
      final GnuPlotRenderer renderer = GnuPlotRenderer.createForGnuPlotExecutable(gnuPlotBin);
      final HeatMap heatMap = HeatMap.builder(data).setTitle("This is a heat map").build();
      final File tmpDir = Files.createTempDir();
      renderer.render(heatMap.renderToEmptyDirectory(tmpDir), new File(tmpDir, "heatmap.png"));
    }
docs: test code