I've written some script to allow for this at least in line charts only allowing zoom on xAxis. The code isn't the best, but it works, would be better if it was in the API.
$("#container").bind('keydown', function () {

let chart = $("#container").highcharts().xAxis[0];

var min = chart.getExtremes().min;

var max = chart.getExtremes().max;

if (event.keyCode == 107 || event.keyCode == 187) {

    //zoom in

    min++;

    max--;

}

if (event.keyCode == 109 || event.keyCode == 189) {

    //zoom out

    if (min > chart.dataMin) {

        min--;

    }

    if (max < chart.dataMax) {

        max++;

    }


}

if (event.keyCode == 39) {

    //right arrow

    if (max < chart.dataMax) {

        min++;

        max++;

    }

}

if (event.keyCode == 37) {

    //left arrow

    if (min > chart.dataMin) {

        min--;

        max--;

    }

}

chart.setExtremes(min, max);

});

This suggestion is an outcome of my longtime involvement with the modelling of many-to-many communications, initially at the RAND Corporation and subsequently at the University of Oxford with some intriguing results in applications such as predicting voter behaviour (see this article http://analytics-magazine.org/voter-motives-and-messages/).

Recently, I found myself involved in the objective analysis for the determination of the best e-banking UI; as it is apparent that soon enough the best bank will be the one with the best UX/UI design.

Here we used highcharts to visualise our findings. Corellating for example the Use Case scores vs. the Functionalities available using a variety of charts (doughnut, barcharts, sunburst, area maps etc). As we strived to make our own UI better, we found imperative to sychronise/drill in, to multiple charts in one go. At times, to drill in to charts that had different datasets.

This effort could have been impossible if it wasn't for the superb help we received from highchart's support guys. However, we soon realised that what the support guys did for us, was over and above, what the product delivers. It was mere, unsupported, customisations made possible due to the great quality of the charts engine.

It will be great if a new version of highcharts also comes with an API that will allow us to easily co-oridinate the synchronisation of different charts with different datasets, that share the same initial options, or even mere labels with ease.

As the charts will interact with one another it is bound to provide us with even better ideas on the optimisation of the visualisation of our data. This is bound to have great, impossible to quantify impact; hopefully similar to the impact that mass many-to-many interaction between people is having in the past two decades.

Unsure of the terms here, but currently the only way to make the mouseover scan vertically instead of horizontally is by inverting the chart. This however inverts the Highstock navigator as well, making the chart I need vertical mouseover scan for look strange compared to the other charts on my site. The solution suggested by highsoft support is to make a second chart which resembles the navigator , but is too time-expensive to impement on every chart on my site..

So, I either wish for an option to flip the mouseover scan without inverting the chart, or an option to place the navigator on what ever side of the chart I please,

Example: https://jsfiddle.net/n7r9yzsu/