modify:解决选中曲线后卡断的问题

src/qcustomplot.cpp

@@ -24536,7 +24536,12 @@ double QCPGraph::pointDistance(const QPointF &pixelPoint, QCPGraphDataContainer:
     {
         // line displayed, calculate distance to line segments:
         QVector<QPointF> lineData;
-        getLines(&lineData, QCPDataRange(0, dataCount())); // don't limit data range further since with sharp data spikes, line segments may be closer to test point than segments with closer key coordinate
+        // 中文注释：曲线点击命中测试原先会为每一条曲线生成整段可见折线，数据量和曲线数较大时单次选中会明显卡顿。
+        // 这里复用上面已经按鼠标选择容差换算出的 key 区间，并依赖 findBegin/findEnd 的 expandedRange 保留边界相邻点；
+        // 这样仍能覆盖穿过鼠标附近的线段，同时把一次命中测试的计算量限制在光标附近的数据窗口内。
+        const int beginIndex = int(begin - mDataContainer->constBegin());
+        const int endIndex = int(end - mDataContainer->constBegin());
+        getLines(&lineData, QCPDataRange(beginIndex, endIndex));
         QCPVector2D p(pixelPoint);
         const int step = mLineStyle == lsImpulse ? 2 : 1; // impulse plot differs from other line styles in that the lineData points are only pairwise connected
         for (int i = 0; i < lineData.size() - 1; i += step)