Accepted answer

You can can update each series on one line, but only make the last one redraw the chart like so:

chart.series[0].addPoint([x, 5], false, true);
chart.series[1].addPoint([x, 5], false, true);
chart.series[2].addPoint([x, 5], false, true);
chart.series[3].addPoint([x, 5], true, true);

Here is a JSFiddle illustrating the concept:

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