Accepted answer

The reason of the movement in the black line is the redraw and the animation function of the chart. For some reason the first series(blue line) doesn't show animation after calling addPoint so you don't see the movement. The second parameter in the addPoint function is redraw. Setting it to false for the second series(black line) will stop the movement when updating the points:

series1.addPoint([x, y], false, true);

Here's the DEMO.

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