diff --git a/c/set-protocol/include/set_signal.h b/c/set-protocol/include/set_signal.h
index 0816e77..a27fcd2 100644
--- a/c/set-protocol/include/set_signal.h
+++ b/c/set-protocol/include/set_signal.h
@@ -10,6 +10,12 @@ enum { SET_SIGNAL_POLYNOMIAL, SET_SIGNAL_LINEAR, SET_SIGNAL_PCHIP, SET_SIGNAL_SP
  * Inputs may be unsorted; duplicate times are averaged. No extrapolation.
  * meta[0..2] = original count, unique count, RMSE at original measurements.
  * endpoint=1 includes the last time; 0 samples a cyclic period [first,last).
+ * On success, workspace blocks used by host formula annotations are:
+ * work[0..2*count): sorted original interleaved x,y pairs;
+ * work[2*count..3*count): unique x normalized to [0,1];
+ * work[3*count..4*count): unique averaged y divided by max(1,max(abs(y)));
+ * work[4*count..5*count): PCHIP first / spline second derivatives in these
+ * normalized coordinates (only meta[1] entries in each unique-data block).
  * Return: 0 success, 1 bounds, 2 nonfinite/degenerate data, 3 rank deficient. */
 PCAN_ABI_API int set_signal_reconstruct(const double *x, const double *y, size_t count,
     int method, unsigned degree, size_t output_count, int endpoint,
diff --git a/python/set_devices/plot_formula.py b/python/set_devices/plot_formula.py
new file mode 100644
index 0000000..ac4c8a2
--- /dev/null
+++ b/python/set_devices/plot_formula.py
@@ -0,0 +1,107 @@
+"""Local polynomial representation of the curves actually drawn on a plot."""
+from dataclasses import dataclass
+import math
+
+
+@dataclass(frozen=True)
+class FormulaPiece:
+    left: float
+    right: float
+    coefficients: tuple  # ascending powers of u=(x-left)/(right-left)
+
+    def value(self, x):
+        u = (x - self.left) / (self.right - self.left)
+        value = 0.
+        for coefficient in reversed(self.coefficients):
+            value = value * u + coefficient
+        return value
+
+    def text(self, origin=0., unit=''):
+        def number(value):
+            return format(value, '.6g')
+        terms = []
+        for power, coefficient in enumerate(self.coefficients):
+            if coefficient == 0 and power:
+                continue
+            term = number(abs(coefficient))
+            if power:
+                term += '·u' + ('²' if power == 2 else '³' if power == 3 else f'^{power}' if power > 1 else '')
+            terms.append(('−' if coefficient < 0 else '+' if terms else '') + term)
+        left = self.left - origin
+        shift = ('−' + number(left)) if left >= 0 else ('+' + number(-left))
+        return ('y = ' + ' '.join(terms) + '\n'
+                + f'u = (x{shift})/{number(self.right - self.left)}; '
+                + f'{number(left)} ≤ x ≤ {number(self.right - origin)} {unit}').rstrip()
+
+
+def linear_piece(timestamps, values, x):
+    """A source polyline has a linear equation, not an inferred analytic model."""
+    previous = None
+    for stamp, value in zip(timestamps, values):
+        if not math.isfinite(stamp) or not math.isfinite(value):
+            previous = None
+            continue
+        if previous is not None:
+            t0, y0 = previous
+            if t0 <= x <= stamp and stamp > t0:
+                return FormulaPiece(t0, stamp, (y0, value - y0))
+        previous = stamp, value
+    return None
+
+
+def polynomial_coefficients(nodes, values):
+    """Convert evaluations of an existing polynomial to power coefficients."""
+    divided = list(values)
+    for order in range(1, len(nodes)):
+        for i in range(len(nodes) - 1, order - 1, -1):
+            divided[i] = (divided[i] - divided[i - 1]) / (nodes[i] - nodes[i - order])
+    result = [divided[-1]]
+    for i in range(len(nodes) - 2, -1, -1):
+        product = [0.] * (len(result) + 1)
+        for power, coefficient in enumerate(result):
+            product[power] -= nodes[i] * coefficient
+            product[power + 1] += coefficient
+        product[0] += divided[i]
+        result = product
+    return tuple(result)
+
+
+def reconstruction_pieces(work, count, unique_count, method, degree, output_y, endpoint):
+    """Read the native reconstruction workspace; no second interpolation fit.
+
+    set_signal.c stores sorted original pairs, normalized x/y and derivatives
+    in the first five count-sized blocks. Derivatives are with respect to the
+    normalized domain, and spline derivatives are second derivatives.
+    """
+    origin, end = work[0], work[2 * (count - 1)]
+    span = end - origin
+    scale = 1.
+    i = 0
+    while i < count:
+        j = i + 1
+        while j < count and work[2 * j] == work[2 * i]:
+            j += 1
+        scale = max(scale, abs(sum(work[2 * k + 1] / (j - i) for k in range(i, j))))
+        i = j
+    if method == 'polynomial':
+        indices = [round(i * (len(output_y) - 1) / degree) for i in range(degree + 1)]
+        nodes = [i / (len(output_y) - (1 if endpoint else 0)) for i in indices]
+        coefficients = polynomial_coefficients(nodes, [output_y[i] for i in indices])
+        return (FormulaPiece(origin, end, coefficients),)
+    pieces = []
+    for i in range(unique_count - 1):
+        x0, x1 = work[2 * count + i], work[2 * count + i + 1]
+        y0, y1 = work[3 * count + i], work[3 * count + i + 1]
+        d0, d1 = work[4 * count + i], work[4 * count + i + 1]
+        h = x1 - x0
+        if method == 'linear':
+            coefficients = (y0, y1 - y0)
+        elif method == 'pchip':
+            coefficients = (y0, h * d0, 3 * (y1 - y0) - h * (2 * d0 + d1),
+                            2 * (y0 - y1) + h * (d0 + d1))
+        else:
+            coefficients = (y0, y1 - y0 - h * h * (2 * d0 + d1) / 6,
+                            h * h * d0 / 2, h * h * (d1 - d0) / 6)
+        pieces.append(FormulaPiece(origin + span * x0, origin + span * x1,
+                                   tuple(c * scale for c in coefficients)))
+    return tuple(pieces)
diff --git a/python/set_devices/plot_processing.py b/python/set_devices/plot_processing.py
index 1b626cc..afd5a78 100644
--- a/python/set_devices/plot_processing.py
+++ b/python/set_devices/plot_processing.py
@@ -76,6 +76,13 @@ class Curve:
     input_count: int
     unique_count: int
     rmse: float
+    pieces: tuple = ()
+
+    def piece_at(self, x):
+        for piece in self.pieces:
+            if piece.left <= x <= piece.right:
+                return piece
+        return None
 
     @property
     def label(self):
@@ -100,8 +107,8 @@ def prepare(snapshot, key, method="pchip", output_count=1000, degree=2):
 def process(request):
     if request is None:
         raise ValueError("Нет доступного аналогового канала")
-    result = reconstruct(request.series.points, request.method, request.output_count, request.degree)
-    return Curve(request, tuple(result.points), result.input_count, result.unique_count, result.rmse)
+    result = reconstruct(request.series.points, request.method, request.output_count, request.degree, with_model=True)
+    return Curve(request, tuple(result.points), result.input_count, result.unique_count, result.rmse, result.pieces)
 
 
 def write_csv(curve, stream):
diff --git a/python/set_devices/qt_ports/plot_annotations.py b/python/set_devices/qt_ports/plot_annotations.py
new file mode 100644
index 0000000..44529b8
--- /dev/null
+++ b/python/set_devices/qt_ports/plot_annotations.py
@@ -0,0 +1,28 @@
+"""Small right-aligned mathematical annotations shared by Qt plots."""
+try:
+    from PySide6.QtCore import QRectF, Qt
+    from PySide6.QtGui import QColor
+except ImportError:
+    from PySide2.QtCore import QRectF, Qt
+    from PySide2.QtGui import QColor
+
+
+def paint_formulas(painter, rect, entries):
+    """Draw (color, text) entries in the right half, wrapping long formulas."""
+    painter.save()
+    painter.setClipRect(rect)
+    metrics = painter.fontMetrics()
+    width = max(1., rect.width() * .48 - 12)
+    top = rect.top() + 5
+    flags = int(Qt.AlignRight | Qt.AlignTop | Qt.TextWordWrap)
+    for color, text in entries:
+        box = QRectF(rect.right() - width - 6, top, width, max(1., rect.bottom() - top))
+        height = painter.boundingRect(box, flags, text).height()
+        box.setHeight(height)
+        painter.fillRect(box.adjusted(-3, -1, 3, 1), QColor('#0b1119'))
+        painter.setPen(QColor(color))
+        painter.drawText(box, flags, text)
+        top += height + metrics.height() * .4
+        if top >= rect.bottom():
+            break
+    painter.restore()
diff --git a/python/set_devices/qt_ports/plot_processing.py b/python/set_devices/qt_ports/plot_processing.py
index 25c8ac3..307a95d 100644
--- a/python/set_devices/qt_ports/plot_processing.py
+++ b/python/set_devices/qt_ports/plot_processing.py
@@ -332,6 +332,17 @@ class PlotProcessingAttachment(QObject):
             self._dirty = True
             self._refresh_timer.start(0)
 
+    def formula_entries(self, x, origin=0., unit=''):
+        entries = []
+        offset = self._external_offset if self._external_curve is not None else 0.
+        for curve in self.curves:
+            piece = curve.piece_at(x + offset)
+            if piece is not None:
+                color_index = self.panel._result_ids[id(curve)] if self._external_curve is None else 0
+                text = curve.label + '\n' + piece.text(origin + offset, unit)
+                entries.append((RESULT_COLORS[color_index % len(RESULT_COLORS)], text))
+        return entries
+
     def refresh(self):
         if self.panel is not None:
             self.panel.set_snapshot(self.snapshot())
diff --git a/python/set_devices/signal_reconstruction.py b/python/set_devices/signal_reconstruction.py
index 776d6ee..3172526 100644
--- a/python/set_devices/signal_reconstruction.py
+++ b/python/set_devices/signal_reconstruction.py
@@ -14,6 +14,7 @@ class Reconstruction:
     input_count: int
     unique_count: int
     rmse: float
+    pieces: tuple = ()
 
 @lru_cache(maxsize=1)
 def library():
@@ -29,7 +30,7 @@ def library():
     except (AttributeError, OSError, RuntimeError) as error:
         raise ValueError("Пересоберите SETProtocol с set_signal.c и set_wavegen.c") from error
 
-def reconstruct(points, method="pchip", output_count=1000, degree=2, *, endpoint=True):
+def reconstruct(points, method="pchip", output_count=1000, degree=2, *, endpoint=True, with_model=False):
     if method not in METHODS or type(output_count) is not int or not 2 <= output_count <= 10000:
         raise ValueError("Неизвестный метод или число выходных точек вне 2…10000")
     count = len(points)
@@ -44,7 +45,14 @@ def reconstruct(points, method="pchip", output_count=1000, degree=2, *, endpoint
         raise ValueError({1: "Недостаточно точек для выбранной степени или неверные параметры",
                           2: "Нужны конечные значения и минимум две различные временные точки",
                           3: "Неустойчивая аппроксимация: уменьшите степень"}.get(code, "Ошибка расчёта"))
-    return Reconstruction(list(zip(ox, oy)), int(meta[0]), int(meta[1]), meta[2])
+    pieces = ()
+    if with_model:
+        from .plot_formula import reconstruction_pieces
+        if method == 'polynomial' and output_count <= degree:
+            pieces = reconstruct(points, method, degree + 1, degree, with_model=True).pieces
+        else:
+            pieces = reconstruction_pieces(work, count, int(meta[1]), method, degree, oy, endpoint)
+    return Reconstruction(list(zip(ox, oy)), int(meta[0]), int(meta[1]), meta[2], pieces)
 
 def dac12(volts, vref=3.3):
     data = (C.c_double * len(volts))(*volts)
diff --git a/python/tests/test_plot_formula.py b/python/tests/test_plot_formula.py
new file mode 100644
index 0000000..e30915d
--- /dev/null
+++ b/python/tests/test_plot_formula.py
@@ -0,0 +1,32 @@
+"""Formula coefficients must reproduce the native reconstruction, not a refit."""
+import unittest
+from set_devices.signal_reconstruction import reconstruct
+from set_devices.plot_formula import linear_piece
+
+
+class PlotFormulaTests(unittest.TestCase):
+    def test_all_methods_match_native_values_on_irregular_and_duplicate_knots(self):
+        points = [(0, 2), (.3, 8), (.3, 4), (1.1, -3), (2, 7), (4, 1), (5, 2)]
+        for method in ('linear', 'pchip', 'spline', 'polynomial'):
+            result = reconstruct(points, method, 101, 3, with_model=True)
+            for x, y in result.points:
+                piece = next(piece for piece in result.pieces if piece.left <= x <= piece.right)
+                self.assertAlmostEqual(y, piece.value(x), places=9)
+
+    def test_polynomial_model_with_fewer_output_samples_than_coefficients(self):
+        points = [(i, 1 + i * .2 - i ** 2 * .03 + i ** 5 * .0001) for i in range(9)]
+        result = reconstruct(points, 'polynomial', 2, 5, with_model=True)
+        for x, y in points:
+            self.assertAlmostEqual(y, result.pieces[0].value(x), places=9)
+
+    def test_source_line_and_formula_use_displayed_origin(self):
+        piece = linear_piece([1000, 1100, 1200], [1, 3, 0], 1050)
+        self.assertAlmostEqual(2, piece.value(1050))
+        text = piece.text(1000, 'мс')
+        self.assertIn('y = 1 +2·u', text)
+        self.assertIn('0 ≤ x ≤ 100 мс', text)
+        self.assertIsNone(linear_piece([0, 1, 2], [1, float('nan'), 3], .5))
+
+
+if __name__ == '__main__':
+    unittest.main()