Nearest Valley Marker
class NearestValleyMarker(var traceIndex: Int, var name: String, var x: Double, var upperYBound: Double? = null, var lowerXBoundMarker: String? = null, var upperXBoundMarker: String? = null, var referenceMarker: String? = null) : Overlay
Puts a marker on the trace at the nearest valley to a fixed x position