QCustomPlot之颜色图或热力图（QCPColorMap）实例1：图形横跨正负坐标轴（x、y）

// configure axis rect:配置轴矩形
customPlot->setInteractions(QCP::iRangeDrag|QCP::iRangeZoom); // 这也将允许通过拖拽/缩放尺度改变颜色范围
customPlot->axisRect()->setupFullAxesBox(true);
customPlot->xAxis->setLabel("x");
customPlot->yAxis->setLabel("y");
 
// set up the QCPColorMap:
QCPColorMap *colorMap = new QCPColorMap(customPlot->xAxis, customPlot->yAxis);
int nx = 200;
int ny = 200;
colorMap->data()->setSize(nx, ny); // 我们希望彩色地图有nx*ny的数据点
colorMap->data()->setRange(QCPRange(-4, 4), QCPRange(-4, 4)); // 并在键（x）和值（y）维上跨越坐标范围-4..4
// ：现在，我们通过访问颜色贴图的QCPColorMapData实例来分配一些数据：
double x, y, z;
for (int xIndex=0; xIndex<nx; ++xIndex)
{
  for (int yIndex=0; yIndex<ny; ++yIndex)
  {
    colorMap->data()->cellToCoord(xIndex, yIndex, &x, &y);//这个函数是为了获得X值
    double r = 3*qSqrt(x*x+y*y)+1e-2;
    z = 2*x*(qCos(r+2)/r-qSin(r+2)/r); // the B field strength of dipole radiation (modulo physical constants)
    colorMap->data()-&g