Swift 4 Charts Can't set the ChartYAxis Domain with Negative Numbers - ios

Question

I'm struggling with the modifiers in the Swift 4 Chart API. I'm attempting to make
a RuleMark Chart where some of the data are negative numbers.
When I set the chart domain to 0...whatever, the chart is presented, but the "bottom"
of the chart is above the minimum values. If I attempt to put a negative number in
the domain range, the code breaks. The diagnostics say "ambiguous use of operator '-'"
If I do not include a .chartYScale, the chart is correctly presented, but the Swift
assigned y scale values leave a lot of wasted empty space making the chart unfriendly.
I've tried many range/domain options but have been unsuccessful. I want to be able to
programmatically control the Y scale including a negative number range.
Here's the Chart view:
struct RuleChartStandard: View {
let dataStore = DataStore.shared
var body: some View {
Chart(dataStore.dayInfoRecords) { dayInfo in
RuleMark(
x: .value("Date", dayInfo.dateString),
yStart: .value("Temperature", dayInfo.high),
yEnd: .value("Temperature", dayInfo.low)
)
}
.frame(height: 500)
.padding(.leading, 25)
.chartYScale(domain: 0...125)//this works, but the scale is incorrect
//.chartYScale(domain: -50...125)//this does not work
}
}
And here is the helper dataStore file:
class DataStore: ObservableObject {
static let shared = DataStore()
#Published var dayInfoRecords: [DayInfo] = []
init() {
generateDayInfoRecords(number: 50)
}//init
func generateDayInfoRecords(number: Int) {
for x in 0..<number {
let di = DayInfo(date: generateDateForDayInfo(dayIndex: x), precipitation: Double.random(in: 0..<10), snowfall: Double.random(in: 0..<10), high: Double.random(in: 45..<110), low: Double.random(in: -30..<45))
dayInfoRecords.append(di)
}
}//generate records
func generateDateForDayInfo(dayIndex: Int) -> Date {
var components = DateComponents()
components.day = dayIndex + 1
components.month = 1
components.year = 2022
let date = Calendar.current.date(from: components)
return date ?? Date()
}
}//class
Any guidance would be appreciated. Xcode 14.2, iOS 16.2

Answer 1

Credit Joakim Danielson with pointing out that it is the Preview that is misbehaving. The negative number domain bottom works in a simulator or device.
And for others to take this further, you can specify the stride amount like this:
var yAxisValues: [Int] {
stride(from: -30, to: 120, by: 10).map { $0 }
}
Then add the modifier to the Chart:
.chartYAxis {
AxisMarks(values: yAxisValues)
}