The chart's value axis can be zoomable. To enable zooming of the value axis, simply add valueScrollbar into your chart config.
If you also want the value axis to load pre-zoomed to certain values, use chart's rendered event handler to call value axis' zoomToValues() function.
"listeners": [{
"event": "rendered",
"method": function(e) {
e.chart.valueAxes[0].zoomToValues(30, 70);
}
}]