https://code.google.com/p/openhab/source/browse/bundles/persistence/org.openhab.persistence.rrd4j/src/main/java/org/openhab/persistence/rrd4j/internal/charts/RRD4jChartServlet.java

RRD4jChartServlet.java

        public void service(ServletRequest req, ServletResponse res)
                        throws ServletException, IOException {
                RrdGraphDef graphDef = new RrdGraphDef();

                configureImageParameters(graphDef, req);
                configureContents(graphDef, req);

                try {
                        RrdGraph graph = new RrdGraph(graphDef);
                        BufferedImage bi = new BufferedImage(graph.getRrdGraphInfo().getWidth(), graph.getRrdGraphInfo().getHeight(), BufferedImage.TYPE_INT_RGB);
                        graph.render(bi.getGraphics());
                        res.setContentType("image/png");
                        javax.imageio.ImageIO.write(bi, "png", res.getOutputStream());
                } catch(FileNotFoundException e) {
                        throw new ServletException("Could not read database files for all requested items.", e);
                }

        }

servlet to write RRD4j graphs directly to a client.