Swift Charts LineChart: Display day on x-axis even if the respective γ-value is nill

It is my first time using the native Swift Charts framework and I ran into a problem regarding my x-axis of my chart.

First, let me tell you about the chart behaviour I want to achieve:

I want to render a chart that displays weight data of the last seven days (including today), similarly to how the Apple Health app does this in the "week-tab". From the screenshot of Apple Health you can see that the x-Axis always has the last seven days on it, irrespective whether there actually is data for the displayed days. In the left screenshot you see a full week in which all days have valid data. In the right screenshot you see a full week but for the last day ("today") I removed the data. Apple Health still rightly shows the day on the x-axis, it just doesn't use the data point for the LineChart, keeping the x-axis to a fixed 7 days:

Desired x-axis behaviour in Apple Health app:

Here is my attempt to re-create the described chart behavior:

Every chart data element has a .date and optinally an .averageWeight:

struct WeightChartData: Identifiable
{
    var id = UUID()
    var date: Date
    var averageWeight: Double?
}

I use a data array containing 7 such WeightChartData elements, representing the users last 7 days of weight data (including today). Weight data on any day might be nill (when no HealthKit data was retrieved for that day). The array is populated with data so that the last element represents "today". Here is my testing/debugging test array (so you can better see the structure of my actual data array):

var testData: [WeightChartData] =
[
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -6, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: 95.1),
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -5, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: 94.2),
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -4, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: 94.4),
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -3, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: nil),
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -2, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: 93.5),
    WeightChartData(date: Calendar.current.date(byAdding: .day, value: -1, to: Calendar.current.startOfDay(for: Date()))!, averageWeight: 93.6),
    WeightChartData(date: Calendar.current.startOfDay(for: Date()),                                                        averageWeight: nil)
]

It is important to note that the array might include WeightChartData-elements with .averageWeight == nill, meaning no weight data exists on those days.

This is how I attempted to draw the chart (WeightViewModel.Data is my "production" data array containing 7 WeightChartData-elements). Note the conditional LineMark(...):

Chart(WeightViewModel.Data)
{ item in
	if item.averageWeight != nil
	{
		LineMark(
			x: .value("Date", item.date, unit: .weekday),
			y: .value("Value", item.averageWeight!)
		)
	}
}

I use these two modifiers to ensure my y-Axis scale is around the min and max .averageWeight of the data array.

.chartYScale(
	domain: [ getBottomRange(), getTopRange() ]
)

.chartYAxis
{
	AxisMarks(
		position: .leading,
		values: .automatic(desiredCount: getYMarkCount())
	)
}

This (γ-axis-customization) works as expected. Still, here are getBottomRange(), getTopRange() and `getYMarkCount() if you think these might cause my x-axis-related issue:

func getTopRange() -> Int
{
	return Int(round(WeightViewModel.maxWeightData()?.averageWeight ?? 0.0) + 1.0)
}
    
func getBottomRange() -> Int
{
	return Int(round(WeightViewModel.minWeightData()?.averageWeight ?? 0.0) - 1.0)
}

func getYMarkCount() -> Int
{        
	return abs(getTopRange() - getBottomRange())
}

For the x-axis, I use this to create one AxisMark(...) for every day (so 7 elements on the x-Axis, irrespective of the elements containing weight data or nil, like in Apple Health):

.chartXAxis
{
	AxisMarks(values: .stride(by: .day))
	{ date in
		AxisGridLine()
		AxisValueLabel(format: .dateTime.weekday(.narrow), centered: true)
	}
}

Let me now show you what happen's if one day has .averageWeight == nil:

• If all of WeightViewModel.Data elements have valid .averageWeight: the graph draws correctly (meaning drawing all 7 days on the x-Axis):

• If an element (that is not the first or last element) has .averageWeight == nil, the chart rightly skips this y-value while stil showing the day on the x-Axis:

like in Apple Health. In this example only the second to last element
has .averageWeight == nil.

• The problem is when the first or last element’s .averageWeight == nil. Again, because the LineMark(..) is only called if item.averageWeight != nil, this first or last data point is skipped. Only this time, because it’s the first or last element, my x-Axis scale gets disrupted. Instead of still showing 7 days, it will now remove the day that has no weightData from the chart, resulting in only having 6 days on the x-Axis:

In this example I set the last element's .averageWeightto nil.
Note that Thursday is no longer on the x-Axis.

This is not all too surprising, as when the first or last element get’s skippid, the chart doesn’t even know that it has to display 7 dates. But I still want the day (that has no .averageWeight) to be shown on the chart’s x-Axis (as my x-Axis should always have 7 elements representing the last 7 days).

How would one achieve the described chart behavior of always showing the full week even when having optional data that might be nil?

Thank you for reading all of this and trying to help me.

Like I said, I'm only using Swift/SwiftUI a couple of weeks now, but apparently you can use .chartXScale(domain:) with a closed date range...

> domain:
The possible data values along the x axis in the chart. You can define the domain with a ClosedRange for number or Date values (e.g., 0 ... 500) . . . - Documentation

I got my intended behavior oddly only after adding one day after the last element using:

.chartXScale(domain: WeightViewModel.Data.first!.date ... Calendar.current.date(byAdding: .day, value: 1, to: WeightViewModel.Data.last!.date)!