Chart series were paired by array index (n = min(data1.size(), data2.size())), silently comparing prices from different calendar dates whenever trading days diverge between symbols.

Changes

• Date-aligned series: Build Map<LocalDate, PriceData> for each symbol and compute the intersection of dates before plotting — every x-coordinate now maps to the same calendar date for both symbols
• Duplicate date handling: toMap uses a merge function (existing, replacement) -> existing to avoid IllegalStateException on duplicate dates in a series
• Empty intersection guard: Returns an error to the user if no overlapping trading dates exist in the selected range

// Before — index-based, silently misaligns on missing days
int n = Math.min(data1.size(), data2.size());
for (int i = 0; i < n; i++) {
    PriceData p1 = data1.get(i);  // may be a different date than p2
    PriceData p2 = data2.get(i);
    ...
}

// After — date-intersection aligned
Map<LocalDate, PriceData> map1 = data1.stream()
        .collect(Collectors.toMap(PriceData::getDate, p -> p, (e, r) -> e));
Map<LocalDate, PriceData> map2 = data2.stream()
        .collect(Collectors.toMap(PriceData::getDate, p -> p, (e, r) -> e));

Set<LocalDate> commonDates = new LinkedHashSet<>(map1.keySet());
commonDates.retainAll(map2.keySet());
List<LocalDate> sortedDates = new ArrayList<>(commonDates);
sortedDates.sort(Comparator.naturalOrder());

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