use interpolation for higher accuracy

mainly useful for the overall delta's in the NP and last30day charts
however, we may as well display them on the chart as well to make it
more consistent, but in this case, we add a small tooltip indicator to
avoid confusion

Author: Dieterbe

lib/helpers/measurements.dart

@@ -0,0 +1,100 @@
+/*
+ * This file is part of wger Workout Manager <https://github.com/wger-project>.
+ * Copyright (C) 2020, 2021 wger Team
+ *
+ * wger Workout Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import 'package:wger/helpers/misc.dart';
+import 'package:wger/widgets/measurements/charts.dart';
+
+extension MeasurementChartEntryListExtensions on List<MeasurementChartEntry> {
+  List<MeasurementChartEntry> whereDate(DateTime start, DateTime? end) {
+    return where((e) => e.date.isAfter(start) && (end == null || e.date.isBefore(end))).toList();
+  }
+
+  List<MeasurementChartEntry> whereDateWithInterpolation(DateTime start, DateTime? end) {
+    // Make sure our list is sorted by date
+    sort((a, b) => a.date.compareTo(b.date));
+
+    // Initialize result list
+    final List<MeasurementChartEntry> result = [];
+
+    // Check if we have any entries on the same day as start/end
+    bool hasEntryOnStartDay = false;
+    bool hasEntryOnEndDay = false;
+
+    // Track entries for potential interpolation
+    MeasurementChartEntry? lastBeforeStart;
+    MeasurementChartEntry? lastBeforeEnd;
+
+    // Single pass through the data
+    for (final entry in this) {
+      if (entry.date.isSameDayAs(start)) {
+        hasEntryOnStartDay = true;
+      }
+      if (end != null && entry.date.isSameDayAs(end)) {
+        hasEntryOnEndDay = true;
+      }
+
+      if (end != null && entry.date.isBefore(end)) {
+        lastBeforeEnd = entry;
+      }
+
+      if (entry.date.isBefore(start)) {
+        lastBeforeStart = entry;
+      } else {
+        // insert interpolated start value if needed
+        if (!hasEntryOnStartDay && lastBeforeStart != null) {
+          result.insert(0, interpolateBetween(lastBeforeStart, entry, start));
+          hasEntryOnStartDay = true;
+        }
+
+        if (end == null || entry.date.isBefore(end)) {
+          result.add(entry);
+        }
+        if (end != null && entry.date.isAfter(end)) {
+          // insert interpolated end value if needed
+          // note: we only interpolate end if we have data going beyond end
+          // if let's say your plan ends in a week from now, we wouldn't want to fake data until next week.
+          if (!hasEntryOnEndDay && lastBeforeEnd != null) {
+            result.add(interpolateBetween(lastBeforeEnd, entry, end));
+            hasEntryOnEndDay = true;
+          }
+          // we added all our values and did all interpolations
+          // surely all input values from here on are irrelevant.
+          return result;
+        }
+      }
+    }
+    return result;
+  }
+}
+
+// caller needs to make sure that before.date < date < after.date
+MeasurementChartEntry interpolateBetween(
+    MeasurementChartEntry before, MeasurementChartEntry after, DateTime date) {
+  final totalDuration = after.date.difference(before.date).inMilliseconds;
+  final startDuration = date.difference(before.date).inMilliseconds;
+
+  // Create a special DateTime with milliseconds ending in 123 to mark it as interpolated
+  // which we leverage in the UI
+  final markedDate =
+      DateTime(date.year, date.month, date.day, date.hour, date.minute, date.second, 123);
+
+  return MeasurementChartEntry(
+    before.value + (after.value - before.value) * (startDuration / totalDuration),
+    markedDate,
+  );
+}

lib/widgets/measurements/charts.dart

@@ -79,12 +79,17 @@ class _MeasurementChartWidgetFlState extends State<MeasurementChartWidgetFl> {
         getTooltipColor: (touchedSpot) => Theme.of(context).colorScheme.primaryContainer,
         getTooltipItems: (touchedSpots) {
           return touchedSpots.map((touchedSpot) {
-            final DateTime date = DateTime.fromMillisecondsSinceEpoch(touchedSpot.x.toInt());
+            final msSinceEpoch = touchedSpot.x.toInt();
+            final DateTime date = DateTime.fromMillisecondsSinceEpoch(msSinceEpoch);
             final dateStr =
                 DateFormat.Md(Localizations.localeOf(context).languageCode).format(date);
 
+            // Check if this is an interpolated point (milliseconds ending with 123)
+            final bool isInterpolated = msSinceEpoch % 1000 == 123;
+            final String interpolatedMarker = isInterpolated ? ' (interpolated)' : '';
+
             return LineTooltipItem(
-              '$dateStr: ${touchedSpot.y.toStringAsFixed(1)} ${widget._unit}',
+              '$dateStr: ${touchedSpot.y.toStringAsFixed(1)} ${widget._unit}$interpolatedMarker',
               TextStyle(color: touchedSpot.bar.color),
             );
           }).toList();

lib/widgets/measurements/helpers.dart

@@ -61,8 +61,8 @@ List<Widget> getOverviewWidgetsSeries(
     for (final plan in plans)
       ...getOverviewWidgets(
         AppLocalizations.of(context).chartDuringPlanTitle(name, plan.description),
-        entriesAll.whereDate(plan.startDate, plan.endDate),
-        entries7dAvg.whereDate(plan.startDate, plan.endDate),
+        entriesAll.whereDateWithInterpolation(plan.startDate, plan.endDate),
+        entries7dAvg.whereDateWithInterpolation(plan.startDate, plan.endDate),
         unit,
         context,
       ),
@@ -74,8 +74,8 @@ List<Widget> getOverviewWidgetsSeries(
         entriesAll.any((e) => e.date.isAfter(monthAgo)))
       ...getOverviewWidgets(
         AppLocalizations.of(context).chart30DaysTitle(name),
-        entriesAll.whereDate(monthAgo, null),
-        entries7dAvg.whereDate(monthAgo, null),
+        entriesAll.whereDateWithInterpolation(monthAgo, null),
+        entries7dAvg.whereDateWithInterpolation(monthAgo, null),
         unit,
         context,
       ),