drawRect function not triggered when needDisplay is called - swift

Question

I am trying to create a custom class to build various graph views for the app eventually. The main class is the viewController class where I would like to present the graphs managed by the clGraph class. clGraph class will be used to process the data and create a graph view by vGraph class.
plotGraph function in the vGraphs is where I am struggling. It appears that the drawRect function is not called, however, setNeedsDisplay is called. Below is the test code that I have to understand and learn the basics. Requesting help to be able to draw a basic line on the targetView. Thank you.
class main : NSViewController {
#IBOutlet weak var targetView: NSView!
func plotGraph(){
let g = clGraphs()
g.initialize(graphCanvas: targetView, controller: self)
g.processData()
g.plotline()
}
}
class clGraphs {
var controller = NSViewController()
var graphCanvas = NSView()
var lineData = [Float]()
public func initialize (graphCanvas : NSView, controller : NSViewController) -> clGraphs{
self.graphCanvas = graphCanvas
self.controller = controller
return self
}
public func plotline(){
let lb = NSTextField(frame: NSRect(x: 10, y: 10, width: 100, height: 20))
lb.stringValue = "Test" //works fine on the view
graphCanvas.addSubview(lb)
print(graphCanvas.frame) //works fine
let g = vGraphs()
g.graphCanvas = graphCanvas
g.needsDisplay = true
}
public func processData(){
//Some logic to update lineData
}
}
class vGraphs: NSView{
var graphCanvas = NSView()
var lineData = [Float]()
override func draw(_ dirtyRect: NSRect) {
print("In the draw rect function")
}
override func setNeedsDisplay(_ invalidRect: NSRect) {
print("In the set needs display function")
plotGraph()
}
func plotGraph(){
graphCanvas.layer?.backgroundColor = NSColor.blue.cgColor //works fine
let aPath = NSBezierPath()
aPath.move(to: NSPoint(x: 50, y: 50))
aPath.line(to: NSPoint(x: 200, y: 200))
aPath.lineWidth = 10
aPath.close()
aPath.stroke()
// ERROR: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
}
}