SWIFT Syntax question with regards to a CLOSURE - swift

Question

maybe one could be so kind as to explain me this snippet
There is this nice tutorial about Core Graphics on raywenderlich. Unfortunately, the comments on that page are closed
The author declares
//Weekly sample data
var graphPoints = [4, 2, 6, 4, 5, 8, 3]
Note the "s" at the end of graphPoints. Then, to calculate the y coordinate for a chart with such figures, he uses graphPoint (without an "s" at the end) within a closure. Nevertheless the code runs just fine to my confusion.
// calculate the y point
let topBorder = Constants.topBorder
let bottomBorder = Constants.bottomBorder
let graphHeight = height - topBorder - bottomBorder
let maxValue = graphPoints.max()!
let columnYPoint = { (graphPoint: Int) -> CGFloat in
let y = CGFloat(graphPoint) / CGFloat(maxValue) * graphHeight
return graphHeight + topBorder - y // Flip the graph
}
And there is no further use of graphPoint in this project (that I am aware of, using "find"). So I wonder, how are graphPoints with an "s" linked to columnYPoint.
Though I currently have no idea how the y values flow into the closure, let me already extend my question: if my values are in a 2D array with the structure [[x1, x2], [y1, y2]], how would I pass only my y (or only my x) values into this closure?
Cheers!
UPDATE
This is how columnYPoint is used, afterwards, to draw the graph:
// draw the line graph
UIColor.white.setFill()
UIColor.white.setStroke()
// set up the points line
let graphPath = UIBezierPath()
// go to start of line
graphPath.move(to: CGPoint(x: columnXPoint(0), y: columnYPoint(graphPoints[0])))
// add points for each item in the graphPoints array
// at the correct (x, y) for the point
for i in 1..<graphPoints.count {
let nextPoint = CGPoint(x: columnXPoint(i), y: columnYPoint(graphPoints[i]))
graphPath.addLine(to: nextPoint)
}
graphPath.stroke()

Answer 1

As you have correctly identified, this is a closure (put into the variable called columnYPoint, giving it a name):
let columnYPoint = { (graphPoint: Int) -> CGFloat in
let y = CGFloat(graphPoint) / CGFloat(maxValue) * graphHeight
return graphHeight + topBorder - y // Flip the graph
}
So really, it's like a function called columnYPoint:
func columnYPoint(_ graphPoint: Int) -> CGFloat {
let y = CGFloat(graphPoint) / CGFloat(maxValue) * graphHeight
return graphHeight + topBorder - y // Flip the graph
}
Why did the author wrote a closure and put it into a variable, instead of writing a function? I have no idea, because I can't read minds. It's a stylistic choice by the author.
And if you look at how it is being called, this function/closure calculates the Y coordinate of the bar, given the height of the bar, graphPoint. graphPoint is the parameter of the function, so of course it is not used in the rest of the code. As you can see from the caller:
graphPath.move(to: CGPoint(x: columnXPoint(0), y: columnYPoint(graphPoints[0])))
// and
let nextPoint = CGPoint(x: columnXPoint(i), y: columnYPoint(graphPoints[i]))
columnYPoint will be called for each element in graphPoints, so graphPoint will be each value in graphPoints. We need to calculate the coordinates of every bar, after all.
There seems to also be a columnYPoint closure mentioned earlier, which calculates the X coordinate given a given bar index. You can combine these two closures to give you a single closure that gives you a single CGPoint:
let margin = Constants.margin
let graphWidth = width - margin * 2 - 4
let topBorder = Constants.topBorder
let bottomBorder = Constants.bottomBorder
let graphHeight = height - topBorder - bottomBorder
let maxValue = graphPoints.max()!
let columnPoint = { (column: Int, graphPoint: Int) -> CGPoint in
//Calculate the gap between points
let spacing = graphWidth / CGFloat(self.graphPoints.count - 1)
let x = CGFloat(column) * spacing + margin + 2
let y = CGFloat(graphPoint) / CGFloat(maxValue) * graphHeight
return CGPoint(x: x, y: graphHeight + topBorder - y) // Flip the graph
}