Function calculateCategoryInterval

libs/echarts/echarts.simple.js:38432–38523

* Calculate interval for category axis ticks and labels. * To get precise result, at least one of `getRotate` and `isHorizontal` * should be implemented in axis.

(axis)

 * should be implemented in axis.
 */
function calculateCategoryInterval(axis) {
    var params = fetchAutoCategoryIntervalCalculationParams(axis);
    var labelFormatter = makeLabelFormatter(axis);
    var rotation = (params.axisRotate - params.labelRotate) / 180 * Math.PI;

    var ordinalScale = axis.scale;
    var ordinalExtent = ordinalScale.getExtent();
    // Providing this method is for optimization:
    // avoid generating a long array by `getTicks`
    // in large category data case.
    var tickCount = ordinalScale.count();

    if (ordinalExtent[1] - ordinalExtent[0] < 1) {
        return 0;
    }

    var step = 1;
    // Simple optimization. Empirical value: tick count should less than 40.
    if (tickCount > 40) {
        step = Math.max(1, Math.floor(tickCount / 40));
    }
    var tickValue = ordinalExtent[0];
    var unitSpan = axis.dataToCoord(tickValue + 1) - axis.dataToCoord(tickValue);
    var unitW = Math.abs(unitSpan * Math.cos(rotation));
    var unitH = Math.abs(unitSpan * Math.sin(rotation));

    var maxW = 0;
    var maxH = 0;

    // Caution: Performance sensitive for large category data.
    // Consider dataZoom, we should make appropriate step to avoid O(n) loop.
    for (; tickValue <= ordinalExtent[1]; tickValue += step) {
        var width = 0;
        var height = 0;

        // Not precise, do not consider align and vertical align
        // and each distance from axis line yet.
        var rect = getBoundingRect(
            labelFormatter(tickValue), params.font, 'center', 'top'
        );
        // Magic number
        width = rect.width * 1.3;
        height = rect.height * 1.3;

        // Min size, void long loop.
        maxW = Math.max(maxW, width, 7);
        maxH = Math.max(maxH, height, 7);
    }

    var dw = maxW / unitW;
    var dh = maxH / unitH;
    // 0/0 is NaN, 1/0 is Infinity.
    isNaN(dw) && (dw = Infinity);
    isNaN(dh) && (dh = Infinity);
    var interval = Math.max(0, Math.floor(Math.min(dw, dh)));

    var cache = inner$6(axis.model);
    var axisExtent = axis.getExtent();