Coreplot CPTXYGraph plot not displaying correctly - swift

Question

I am using the latest version of Core Plot within a Swift program. The graph plots without a problem and shows the following
If you look at the plot the lower portion of the gradient fill for the plot does not match the xAxis which is the behaviour that I expect. instead it starts part way up the yAxis.
I have not experienced this behaviour before and am not sure where to hunt for the bug in my code. I haven't been able to find a relevant entry elsewhere within stackOverflow or elsewhere on the net for that matter.
I have tested the plotspace variables and they are on the money, min/max for the Y axis is as expected and the X/Y coords through the delegate functions shown below [number() and numberOfRecords()] are as expected. I am not sure what within core plot will be driving this behaviour.
If anyone is able to suggest other areas to examine for the error that would be greatly appreciated.
Rather than post all of the code initially, if someone can suggest an area to explore I will add that element of the code in to the question to help with the diagnosis. I have initially only included the code for the section of the app that defines the scatter line plot [configureLineGraph()] below the delegate functions below.
// Delegate functions for Core Plot
func numberOfRecords(for plot: CPTPlot) -> UInt {
numberOfRecordsToBeDisplayedOnGraph = UInt(arrayOfCommanderProgress.count)
return numberOfRecordsToBeDisplayedOnGraph // this needs to represent the number of records to be displayed on the graph
}
func number(for plot: CPTPlot, field fieldEnum: UInt, record idx: UInt) -> Any? {
let index = Int(numberOfRecordsToBeDisplayedOnGraph - idx) - 1
switch CPTScatterPlotField(rawValue: Int(fieldEnum))! {
case .X:
let xCoord = (arrayOfCommanderProgress[index].timeStamp.convertDateStringToDateObject()).timeIntervalSinceReferenceDate
return xCoord
case .Y:
var yCoord = Double()
if fieldEnum == 1 {
switch graphToDisplay {
case graphCombat:
yCoord = (Double(self.arrayOfCommanderProgress[index].combat)) + (100.0 * (Double(arrayOfRankProgress[index].combat)))
case graphTrade:
yCoord = (Double(self.arrayOfCommanderProgress[index].trade)) + (100.0 * (Double(arrayOfRankProgress[index].trade)))
case graphExploration:
yCoord = (Double(self.arrayOfCommanderProgress[index].explore)) + (100.0 * (Double(arrayOfRankProgress[index].explore)))
case graphFederation:
yCoord = (Double(self.arrayOfCommanderProgress[index].federation)) + (100.0 * (Double(arrayOfRankProgress[index].federation)))
case graphEmpire:
yCoord = (Double(self.arrayOfCommanderProgress[index].empire)) + (100.0 * (Double(arrayOfRankProgress[index].empire)))
case graphCQC:
yCoord = (Double(self.arrayOfCommanderProgress[index].cQC)) + (100.0 * (Double(arrayOfRankProgress[index].cQC)))
default:
break
}
}
return yCoord
}
}
// Funtions to set up the graphView and plots
func configureLineGraph() {
// Plotline variables for scatterplot
let plotLineTotal = CPTScatterPlot()
let areaGradient:CPTGradient = CPTGradient(beginning: CPTColor.clear(), ending: CPTColor(componentRed: 150/255.0, green: 180/255.0, blue: 200/255.0, alpha: 0.9))
areaGradient.angle = 90.0
let areaGradientFill:CPTFill = CPTFill(gradient: areaGradient)
// Set up plot lines for each type of total to be graphed
plotLineTotal.areaFill = areaGradientFill
// set up plotline characteristics
let plotLineType = CPTMutableLineStyle()
plotLineType.lineColor = CPTColor.darkGray()
// establish a reference to the graph custom view
guard let graph = graphView.hostedGraph else { return }
// Add additional plotlines to the plots Array if needed
let plots = [plotLineTotal]
for plot in plots {
plot.dataSource = self
plot.delegate = self
plot.areaBaseValue = CPTDecimalFromInteger(0) as NSNumber
plot.dataLineStyle = plotLineType
graph.add(plot, to: graph.defaultPlotSpace)
}
graph.reloadData()
}