Calculating averages by date from JSON in SwiftUI

Question

I have JSON data that I decoded and stored in core data that I am using to populate a SwiftUi line chart. I need to calculate the average of the data by dates. So the average of polling data by months.

JSON Data:

"president": "Joe Biden",
"subgroup": "All polls",
"modeldate": "9/9/2022",
"startdate": "1/19/2021",
"enddate": "1/21/2021",
"pollster": "Rasmussen Reports/Pulse Opinion Research",
"grade": "B",
"samplesize": 1500,
"population": "lv",
"weight": 0.33818752,
"influence": 0,
"approve": 48,
"disapprove": 45,
"adjusted_approve": 49.154266,
"adjusted_disapprove": 40.2849,
"multiversions": "",
"tracking": "T",
"url": "https://www.rasmussenreports.com/public_content/politics/biden_administration/biden_approval_index_history",
"poll_id": 74247,
"question_id": 139395,
"createddate": "January 2021",
"timestamp": "09:48:31  9 Sep 2022"

Data Model:

struct PresApprovalDTO: Hashable, Decodable {

    var president: String
    var subgroup: String
    var startdate: String
    var enddate: String
    var pollster: String
    var approve: Double
    var disapprove: Double
    var url: String
    var createddate: String
    
    var startDateConverter: Date {
        
        let dateFormatter = DateFormatter()
        dateFormatter.locale = Locale(identifier: "en_US_POSIX") // set locale to reliable US_POSIX
        dateFormatter.dateFormat = "MM-dd-yyyy"
        let date = dateFormatter.date(from:startdate) ?? Date()
        
        let calendar = Calendar.current
        let components = calendar.dateComponents([.year, .month, .day, .hour], from: date)
        
        let finalDate = calendar.date(from:components) ?? Date()
        
        return(finalDate)
    }
    
    var endDateConverter: Date {
        
        let dateFormatter = DateFormatter()
        dateFormatter.locale = Locale(identifier: "en_US_POSIX") // set locale to reliable US_POSIX
        dateFormatter.dateFormat = "MM-dd-yyyy"
        let date = dateFormatter.date(from:enddate) ?? Date()
        
        let calendar = Calendar.current
        let components = calendar.dateComponents([.year, .month, .day, .hour], from: date)
        
        let finalDate = calendar.date(from:components) ?? Date()
        
        return(finalDate)
    }
    
}

Chart View Model:

import SwiftUI
import Charts

struct PresApprChart: View {
    
    @StateObject private var pollListVM = PollsListViewModel()
    
    var body: some View {
        
        Chart(pollListVM.presPolls, id: \.url) { item in
            
            LineMark(x: .value("Month", item.endDateConverter, unit: .month),
                     y: .value("Approval", item.approve)
            )
            
        }.frame(width: 325, height: 200)
            .onAppear(perform: {
                pollListVM.getAllPresPolls()
            })
        
    }
}

struct PresApprChart_Previews: PreviewProvider {
    static var previews: some View {
        PresApprChart()
    }
}

Above is the result. There are tons of data points for each month, so I need to average them. I tried to map and divide the approved values by the date count, but it averages all the approval data points.

        LineMark(x: .value("Month", item.endDateConverter, unit: .month),
                 y: .value("Approval", pollListVM.presPolls.map(\.approve).reduce(0.0, +) / Double(item.endDateConverter.asFormattedString().count))
        )