为什么NVD3 StreamGraph会随着时间推移将y零轴移位？

The NVD3 code for the stacked area chart accepts four possibilities for the chart style: stack, stream, stream-center, and expand. The default stream graph is the "stream" style, and this is the version that gets off-centre if you have any zero values in your data.

If you set the style explicitly to "stream-center", the chart behaves as expected. However, the radio buttons that switch between display styles switch back to the problematic algorithm, so they will need to be disabled.

chart.style('stream-center')
     .showControls(false);

It's possible that the existence of this other display algorithm means that the NVD3 folks are working on it. The github issue page might be a good place to look for updates or responses from them:

https://github.com/novus/nvd3/issues/416