Qwt Plot Curve and Marker Precision with Doubles

Hi,

I am working on a plot where the X axis is a 24 hour clock. Each 1.0 on the X axis equates to 1 second. But I also work with milliseconds, e.g 12.3. Basically, I am trying to place markers on the curve with millisecond precision, however after several zoom levels the markers diverge from the curve coordinates.

A sample of my code is:

for (int i = 0; i < curve->dataSize(); i++)
{
if (12.3 < curve->sample(i).x())
{
xDist = std::abs(curve->sample(i).x() - 12.3);
yDist = std::abs(curve->sample(i).y() - curve->sample(i + 1).y();
yValue = (yDist * xDist) + curve->sample(i).y();
break;
}
}

If there are syntax/spelling mistakes it is probably because I had to type this on my phone. However, this code partly does what I want. As at the highest level it looks perfect, but when I zoom in to values on my y axis with 3 or more decimal places the marker no longer follows the curve, instead hovering around the curve.

Therefore, I am looking for some advice on this. Is there a better/more precise way to find the y value of a curve based on its x axis value? Or is there some way to limit the level of zoom I can apply to a plot? I would prefer the former solution but am happy for any help.

Thanks in advance.