/**
 * Manual pan handler for Chart.js charts with zoom.
 * Enables click-and-drag to pan on the X axis, clamped to data boundaries.
 * Call enablePan(chart, chartId, dataMin, dataMax) after chart creation.
 */
function enablePan(chart, resetBtnId, dataMin, dataMax) {
    const canvas = chart.canvas;
    let isPanning = false;
    let startClientX = 0;
    let startMin = 0;
    let startMax = 0;
    let xScaleWidth = 0;

    canvas.addEventListener('mousedown', (e) => {
        if (e.button !== 0) return;
        isPanning = true;
        startClientX = e.clientX;
        const xScale = chart.scales.x;
        startMin = xScale.min;
        startMax = xScale.max;
        xScaleWidth = xScale.width;
        canvas.style.cursor = 'grabbing';
        e.preventDefault();
        e.stopPropagation();
    });

    const onMove = (clientX) => {
        if (!isPanning) return;
        const xScale = chart.scales.x;
        const pixelDiff = clientX - startClientX;
        const valuePerPixel = (startMax - startMin) / xScaleWidth;
        const shift = -pixelDiff * valuePerPixel;

        let newMin = startMin + shift;
        let newMax = startMax + shift;

        // Clamp to data boundaries
        if (newMin < dataMin) {
            newMin = dataMin;
            newMax = newMin + (startMax - startMin);
        }
        if (newMax > dataMax) {
            newMax = dataMax;
            newMin = newMax - (startMax - startMin);
        }

        // Set via options to persist through update
        // Set via chart.options to persist, and store on chart for next update
        chart.options.scales.x.min = newMin;
        chart.options.scales.x.max = newMax;
        chart._panRange = [newMin, newMax];
        chart.update('none');
    };

    canvas.addEventListener('mousemove', (e) => {
        onMove(e.clientX);
    });

    canvas.addEventListener('mouseup', () => {
        if (isPanning) {
            isPanning = false;
            canvas.style.cursor = '';
            const btn = document.getElementById(resetBtnId);
            if (btn) btn.style.display = 'inline-block';
        }
    });

    canvas.addEventListener('mouseleave', () => {
        if (isPanning) {
            isPanning = false;
            canvas.style.cursor = '';
        }
    });

    // Document-level listeners for drag-outside-canvas
    const docUp = () => {
        if (isPanning) {
            isPanning = false;
            canvas.style.cursor = '';
            const btn = document.getElementById(resetBtnId);
            if (btn) btn.style.display = 'inline-block';
        }
    };
    const docMove = (e) => {
        if (isPanning) onMove(e.clientX);
    };
    document.addEventListener('mouseup', docUp);
    document.addEventListener('mousemove', docMove);
}