As a lousy workaround , I introduced a call to drawchart(true) every some iterations. It seems to release the listeners.

        AtomicInteger iterations = new AtomicInteger();

        Timer timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                if( ui == null ) {
                  return;
                }
                ui.access( () -> {
                    usage_memory_series.add( new DataSeriesItem(
                            Instant.now(), new Random().nextInt(101)), true, true);
                    usage_network_series.add( new DataSeriesItem(
                            Instant.now(), new Random().nextInt(101)), true, true);
                    long t = System.nanoTime();
                    if (iterations.incrementAndGet()==100) {
                        iterations.set(0);
                        usage_chart.drawChart(true);
                    }
                });
            }
        }, 0, 10);

OTOH drawChart() which is equivalent to drawChart(false) does not release listeners:

I my testing, using drawChart(true) does release some listeners and free up some memory, but even adding a drawChart(true) after every update still incurs some memory growth, so that slows the process, but doesn't completely work-around the issue.

This looks like an issue on Highcharts side, as I was able to reproduce what I believe is the same issue in their latest version.

I reported it here, let's see if they confirm it.

As per this comment from one of their maintainers, it has been confirmed that this issue originates on Highcharts.

As usual in this case, we will wait for their fix, and see if it's possible to patch it into our component since Highcharts usually don't backport bug fixes to older versions.

I would assume the fix: vaadin/web-components#7785 will give a new workaround to this issue. Instead of just updating Chart data, throw Chart instance away and replace it by new one?

The changes in vaadin/web-components#7785 could be used as a workaround for this issue, with the downside that removing and attaching a new chart instance would lose the animation after each new data point is added.

The graph shows that the number of listeners stays fairly consistent after running it for over 5 minutes.