QCustomPlot 多曲线，y轴自适应为，最小值的两倍，最大值的两倍

要实现多曲线的y轴自适应，范围为最小值的两倍到最大值的两倍，可以按照以下步骤进行：

1. 获取y轴指针。

QCPAxis *yAxis = ui->customPlot->yAxis;

2. 遍历所有曲线，获取最小值和最大值。

double minY = std::numeric_limits<double>::max();
double maxY = std::numeric_limits<double>::min();
for (int i = 0; i < ui->customPlot->graphCount(); i++) {
    QCPGraph *graph = ui->customPlot->graph(i);
    if (graph->dataCount() == 0) {
        continue;
    }
    QCPGraphDataContainer::const_iterator it = graph->data().constBegin();
    for (; it != graph->data().constEnd(); ++it) {
        double value = it->value;
        if (value < minY) {
            minY = value;
        }
        if (value > maxY) {
            maxY = value;
        }
    }
}

3. 计算自适应的范围，并设置y轴范围。

double range = qMax(qAbs(minY), qAbs(maxY)) * 2;
double center = (minY + maxY) / 2;
yAxis->setRange(center - range, center + range);

完整代码示例：

QCPAxis *yAxis = ui->customPlot->yAxis;
double minY = std::numeric_limits<double>::max();
double maxY = std::numeric_limits<double>::min();
for (int i = 0; i < ui->customPlot->graphCount(); i++) {
    QCPGraph *graph = ui->customPlot->graph(i);
    if (graph->dataCount() == 0) {
        continue;
    }
    QCPGraphDataContainer::const_iterator it = graph->data().constBegin();
    for (; it != graph->data().constEnd(); ++it) {
        double value = it->value;
        if (value < minY) {
            minY = value;
        }
        if (value > maxY) {
            maxY = value;
        }
    }
}
double range = qMax(qAbs(minY), qAbs(maxY)) * 2;
double center = (minY + maxY) / 2;
yAxis->setRange(center - range, center + range);