So basically, I solved this problem by setting the color to transparent each time before the color is re-assigned. Something like this:
for (int i = 0; i < LiveImage.MAX_CHANNELS; i++) // color reset
{
((LineGraph)plotter.Children.ElementAt(startIndex + i)).LinePen = new Pen(new SolidColorBrush(Colors.Transparent), 1);
}
The point is NOT to add any line graph.