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  Get Chart position from mouse position
Posted by Digory on Jun-12-2008 15:23
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There are methods to retrieve the screen position from a given Chart position, i.e. Chart::getXCoor, Chart::getYCoor, or Axis::getCoor. However, there does not seem to be any method that does the reverse thing, i.e. transform a given screen position to the corresponding Chart position (after the Chart has been layouted, of course). As a workaround, I currently use getXCoor, getYCoor and binary search (see below). This works, but I wonder if there is any better way to do this.

BOOL ChartViewer::relPosToXYChartPos (int nRelPosX, int nRelPosY,
   double &dChartPosX, double &dChartPosY, KisDevice &ks)
   {
   if (m_pXYChart != NULL)
      {
      Axis *pXAxis = m_pXYChart->xAxis();
      Axis *pYAxis = m_pXYChart->yAxis();
      if (pXAxis != NULL && pYAxis != NULL)
         {
         double dX1 = pXAxis->getMinValue();
         double dX2 = pXAxis->getMaxValue();
         if (m_pXYChart->getXCoor (dX1) > m_pXYChart->getXCoor (dX2))
            {
            // X-Axis is oriented right to left.
            double dTmp = dX1;
            dX1 = dX2;
            dX2 = dTmp;
            }
         double dY1 = pYAxis->getMinValue();
         double dY2 = pYAxis->getMaxValue();
         if (m_pXYChart->getYCoor (dY1) > m_pXYChart->getYCoor (dY2))
            {
            // Y-Axis is oriented bottom to top.
            double dTmp = dY1;
            dY1 = dY2;
            dY2 = dTmp;
            }
         int nPixelX = ks.lmToTm (nRelPosX);
         int nPixelY = ks.lmToTm (nRelPosY);
         int nToGo = 20; // max number of iterations
         int nTryX, nTryY;
         while (nToGo > 0)
            {
            dChartPosX = (dX1 + dX2) * 0.5;
            dChartPosY = (dY1 + dY2) * 0.5;
            nTryX = m_pXYChart->getXCoor (dChartPosX);
            nTryY = m_pXYChart->getYCoor (dChartPosY);
            if (nTryX == nPixelX && nTryY == nPixelY) break;
            if (nTryX < nPixelX) dX1 = dChartPosX; else dX2 = dChartPosX;
            if (nTryY < nPixelY) dY1 = dChartPosY; else dY2 = dChartPosY;
            nToGo--;
            }
         return TRUE;
         }
      }
   return FALSE;
   }

  Re: Get Chart position from mouse position
Posted by Peter Kwan on Jun-13-2008 01:52
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Hi Digory,

I usually will use interpolation. Only one line of code is need (though the line may be long) per coordinate. The code is like:

double xcoor = (nRelPosX - c->getXCoor(c->xAxis()->getMinValue())) / (c->getXCoor(c->xAxis()->getMaxValue()) - c->getXCoor(c->xAxis()->getMinValue())) * (c->xAxis()->getMaxValue() - c->xAxis()->getMinValue()) + c->xAxis()->getMinValue();

There is some explanation of the code in:

http://www.chartdir.com/forum/download_thread.php?bn=chartdir_support&thread=1187910016#N1187953452

Hope this can help.

Regards
Peter Kwan