[Chartjs]-Chart.js: summing number of items in active/shown datasets on yAxes of pie chart

found it!

options = {
    title: {
        display: true,
        text: 'Stuff'
    },
    scales: {
        yAxes: [{
            ticks: {
                min: 0,
                max: numOfItems,
            },
            beforeUpdate: function (scale) {
                let maxValue = 0
                if (scale.chart.config && scale.chart.config.data && scale.chart.config.data.datasets) {
                    scale.chart.config.data.datasets.forEach(dataset => {
                        if (dataset && dataset.data) {
                            var visibleDataSets = new Array();

                            dataset._meta[1].data.forEach(dataset => {
                                if (dataset.hidden === false) {
                                    visibleDataSets.push(dataset._index);
                                }
                            });

                            visibleDataSets.forEach(index => {
                                maxValue = maxValue + dataset.data[index];
                            });
                        }
                    })
                }

                scale.options.ticks.max = maxValue;

                if (maxValue === 0) {
                    scale.options.ticks.display = false;
                } else {
                    scale.options.ticks.display = true;
                }
            }
        }]
    },
    responsive: true
};

TL;DR dataset._meta[1].data[i].hidden is the answer…