void UpdateFrequencyLayers ()
{
for (int index = 0; index <= gridLineCount; index++) {
var value = GetValue (index);
var location = Math.Floor (GetLocationForFrequencyValue (value));
if (index > 0 && index < gridLineCount) {
freqLines [index - 1].Frame = new CGRect (location, bottomMargin, 1f, graphLayer.Frame.Height);
frequencyLabels [index].Frame = new CGRect (location - labelWidth / 2f, graphLayer.Frame.Height, labelWidth, 16f);
}
frequencyLabels [index].Frame = new CGRect (location - labelWidth / 2f, graphLayer.Frame.Height + 6, labelWidth + rightMargin, 16f);
}
}