loopandlearn · bjorkert · May 30, 2026
@@ -27,6 +27,7 @@ enum GraphDataIndex: Int {
     case smb = 16
     case tempTarget = 17
     case predictionCone = 18
+    case yesterday = 19
 }
 
 extension GraphDataIndex {
@@ -51,6 +52,7 @@ extension GraphDataIndex {
         case .smb: return "SMB"
         case .tempTarget: return "Temp Target"
         case .predictionCone: return "Prediction Cone"
+        case .yesterday: return "Yesterday"
         }
     }
 }
@@ -622,6 +624,16 @@ extension MainViewController {
         lineCone.axisDependency = YAxis.AxisDependency.right
         data.append(lineCone)
 
+        // Dataset 19: Yesterday's BG comparison overlay (thin dimmed gray line, no dots)
+        let lineYesterday = LineChartDataSet(entries: [ChartDataEntry](), label: "")
+        lineYesterday.lineWidth = 1.5
+        lineYesterday.setColor(NSUIColor.systemGray, alpha: 0.4)
+        lineYesterday.drawCirclesEnabled = false
+        lineYesterday.drawValuesEnabled = false
+        lineYesterday.highlightEnabled = false
+        lineYesterday.axisDependency = YAxis.AxisDependency.right
+        data.append(lineYesterday)
+
         data.setValueFont(UIFont.systemFont(ofSize: 12))
 
         // Add marker popups for bolus and carbs
@@ -813,6 +825,11 @@ extension MainViewController {
         BGChart.data?.notifyDataChanged()
         BGChart.notifyDataSetChanged()
 
+        // Reflect the yesterday overlay toggle immediately, and reload the BG window
+        // so the extra day of history is fetched (or dropped) when the toggle changed.
+        updateYesterdayBGGraph()
+        TaskScheduler.shared.rescheduleTask(id: .fetchBG, to: Date())
+
         // Re-render prediction display in case display type changed
         updateOpenAPSPredictionDisplay()
     }
@@ -882,6 +899,8 @@ extension MainViewController {
         BGChartFull.data?.notifyDataChanged()
         BGChartFull.notifyDataSetChanged()
 
+        updateYesterdayBGGraph()
+
         if firstGraphLoad {
             var scaleX = CGFloat(Storage.shared.chartScaleX.value)
             if scaleX > CGFloat(ScaleXMax) {
@@ -899,6 +918,32 @@ extension MainViewController {
         }
     }
 
+    // Populates (or clears) the dimmed "yesterday" comparison overlay on the main graph.
+    // Points in yesterdayBGData are already shifted +24h so they align with today's clock time.
+    func updateYesterdayBGGraph() {
+        let dataIndex = GraphDataIndex.yesterday.rawValue
+        guard let lineData = BGChart.lineData,
+              dataIndex < lineData.dataSets.count,
+              let dataSet = lineData.dataSets[dataIndex] as? LineChartDataSet
+        else {
+            return
+        }
+
+        dataSet.removeAll(keepingCapacity: false)
+
+        if Storage.shared.showYesterdayLine.value {
+            for entry in yesterdayBGData {
+                // Clamp the plotted y-value to the same bounds the main BG line uses.
+                let plottedSgv = Double(min(max(entry.sgv, globalVariables.minDisplayGlucose), globalVariables.maxDisplayGlucose))
+                dataSet.append(ChartDataEntry(x: entry.date, y: plottedSgv))
+            }
+        }
+
+        BGChart.data?.dataSets[dataIndex].notifyDataSetChanged()
+        BGChart.data?.notifyDataChanged()
+        BGChart.notifyDataSetChanged()
+    }
+
     func updatePredictionGraph(color: UIColor? = nil) {
         let dataIndex = 1
         var mainChart = BGChart.lineData!.dataSets[dataIndex] as! LineChartDataSet

@@ -5,11 +5,19 @@ import Foundation
 import UIKit
 
 extension MainViewController {
+    /// Number of days of BG history to request from the source. One extra day is
+    /// added when the "Show Yesterday's BG" overlay is enabled (Nightscout only),
+    /// so the overlay can display the same clock time from the day before.
+    var bgFetchDays: Int {
+        let extraDay = (Storage.shared.showYesterdayLine.value && IsNightscoutEnabled()) ? 1 : 0
+        return Storage.shared.downloadDays.value + extraDay
+    }
+
     // Dex Share Web Call
     func webLoadDexShare() {
         // Dexcom Share only returns 24 hrs of data as of now
         // Requesting more just for consistency with NS
-        let graphHours = 24 * Storage.shared.downloadDays.value
+        let graphHours = 24 * bgFetchDays
         let count = graphHours * 12
         dexShare?.fetchData(count) { err, result in
             if let error = err {
@@ -51,8 +59,8 @@ extension MainViewController {
         }
 
         var parameters: [String: String] = [:]
-        let date = Calendar.current.date(byAdding: .day, value: -1 * Storage.shared.downloadDays.value, to: Date())!
-        parameters["count"] = "\(Storage.shared.downloadDays.value * 2 * 24 * 60 / 5)"
+        let date = Calendar.current.date(byAdding: .day, value: -1 * bgFetchDays, to: Date())!
+        parameters["count"] = "\(bgFetchDays * 2 * 24 * 60 / 5)"
         parameters["find[date][$gte]"] = "\(Int(date.timeIntervalSince1970 * 1000))"
 
         // Exclude 'cal' entries
@@ -207,6 +215,27 @@ extension MainViewController {
         LogManager.shared.log(category: .nightscout,
                               message: "Graph data updated with \(bgData.count) entries.",
                               isDebug: true)
+
+        // Build the optional "yesterday" comparison overlay. Every fetched reading is
+        // shifted +24h so it lines up with the same clock time today; the extra day of
+        // history pulled by bgFetchDays provides the portion that falls inside the
+        // visible window. The overlay is capped to "now + hours of prediction" so it
+        // never extends further into the future than the prediction line.
+        yesterdayBGData.removeAll()
+        if Storage.shared.showYesterdayLine.value, IsNightscoutEnabled() {
+            let cutoff = dateTimeUtils.getTimeIntervalNHoursAgo(N: 24 * bgFetchDays)
+            let futureLimit = dateTimeUtils.getNowTimeIntervalUTC() + Storage.shared.predictionToLoad.value * 3600
+            for i in 0 ..< data.count {
+                let reading = data[data.count - 1 - i]
+                guard reading.date >= cutoff, reading.sgv <= 600 else { continue }
+                let shiftedDate = reading.date + 24 * 60 * 60
+                guard shiftedDate <= futureLimit else { continue }
+                yesterdayBGData.append(ShareGlucoseData(sgv: reading.sgv,
+                                                        date: shiftedDate,
+                                                        direction: reading.direction))
+            }
+        }
+
         viewUpdateNSBG(sourceName: sourceName)
     }
 

@@ -12,6 +12,7 @@ struct GraphSettingsView: View {
     @ObservedObject private var show30MinLine = Storage.shared.show30MinLine
     @ObservedObject private var show90MinLine = Storage.shared.show90MinLine
     @ObservedObject private var showMidnightLines = Storage.shared.showMidnightLines
+    @ObservedObject private var showYesterdayLine = Storage.shared.showYesterdayLine
     @ObservedObject private var smallGraphTreatments = Storage.shared.smallGraphTreatments
 
     @ObservedObject private var smallGraphHeight = Storage.shared.smallGraphHeight
@@ -43,6 +44,9 @@ struct GraphSettingsView: View {
 
                         Toggle("Show −90 min Line", isOn: $show90MinLine.value)
                             .onChange(of: show90MinLine.value) { _ in markDirty() }
+
+                        Toggle("Show Yesterday's BG", isOn: $showYesterdayLine.value)
+                            .onChange(of: showYesterdayLine.value) { _ in markDirty() }
                     }
 
                     Toggle("Show Midnight Lines", isOn: $showMidnightLines.value)

@@ -129,6 +129,7 @@ class Storage {
     var show30MinLine = StorageValue<Bool>(key: "show30MinLine", defaultValue: false)
     var show90MinLine = StorageValue<Bool>(key: "show90MinLine", defaultValue: false)
     var showMidnightLines = StorageValue<Bool>(key: "showMidnightMarkers", defaultValue: false)
+    var showYesterdayLine = StorageValue<Bool>(key: "showYesterdayLine", defaultValue: false)
     var smallGraphTreatments = StorageValue<Bool>(key: "smallGraphTreatments", defaultValue: true)
 
     var smallGraphHeight = StorageValue<Int>(key: "smallGraphHeight", defaultValue: 40)

@@ -77,6 +77,7 @@ class MainViewController: UIViewController, UITableViewDataSource, ChartViewDele
     var profileManager = ProfileManager.shared
 
     var bgData: [ShareGlucoseData] = []
+    var yesterdayBGData: [ShareGlucoseData] = [] // readings already shifted +24h for the comparison overlay
     var basalProfile: [basalProfileStruct] = []
     var basalData: [basalGraphStruct] = []
     var basalScheduleData: [basalGraphStruct] = []