function getSeriesStackId(seriesModel) {
    return seriesModel.get('stack') || STACK_PREFIX + seriesModel.seriesIndex;
}
Map from axis.index to values.
For a single time axis, axisValues is in the form like
{'x_0': [1495555200000, 1495641600000, 1495728000000]}.
Items in axisValues[x], e.g. 1495555200000, are time values of all
series.