diff --git a/c/set-protocol/docs/GUI_PLOT.md b/c/set-protocol/docs/GUI_PLOT.md
index 97f9eda..52234d8 100644
--- a/c/set-protocol/docs/GUI_PLOT.md
+++ b/c/set-protocol/docs/GUI_PLOT.md
@@ -3,7 +3,8 @@
 Численная логика находится в `include/set_plot.h` и `src/set_plot.c`.
 Это модуль C99 без Qt, Android, транспорта, динамической памяти и глобального
 состояния. Он собирается в существующую библиотеку SETProtocol; отдельная DLL
-для графиков не требуется. Версия ABI графиков — `set_plot_abi_version() == 2`.
+для графиков не требуется. Версия ABI графиков — `set_plot_abi_version() == 1`;
+новые операции добавляются без изменения существующих значений и сигнатур.
 
 | Общее в templates | Адаптер приложения |
 |---|---|
diff --git a/c/set-protocol/include/set_plot.h b/c/set-protocol/include/set_plot.h
index df7cc60..8e0b006 100644
--- a/c/set-protocol/include/set_plot.h
+++ b/c/set-protocol/include/set_plot.h
@@ -21,7 +21,7 @@ enum set_plot_operation {
     SET_PLOT_LIMITS = 8     /* xMin,xMax,yMin,yMax -> validated unchanged limits */
 };
 
-/** Version of this plot ABI, independently of the transport ABI (currently 2). */
+/** Version of this additive plot ABI, independently of the transport ABI. */
 PCAN_ABI_API uint32_t set_plot_abi_version(void);
 /** Evaluates one operation. Returns output count, or 0 for invalid arguments.
  * TRANSFORM rejects malformed viewports; invalid gesture values return the
diff --git a/c/set-protocol/src/set_plot.c b/c/set-protocol/src/set_plot.c
index 368275b..ab9d99f 100644
--- a/c/set-protocol/src/set_plot.c
+++ b/c/set-protocol/src/set_plot.c
@@ -8,7 +8,7 @@ static int finite_values(const double *v, size_t n) {
     return 1;
 }
 
-uint32_t set_plot_abi_version(void) { return 2U; }
+uint32_t set_plot_abi_version(void) { return 1U; }
 
 size_t set_plot_eval(uint32_t op, const double *v, size_t n, double *out, size_t cap) {
     static const size_t sizes[] = {10, 3, 4, 4, 6, 2, 3, 2, 4};
diff --git a/c/set-protocol/tests/fixtures/plot-v1.json b/c/set-protocol/tests/fixtures/plot-v1.json
index b5b9a34..a06e11f 100644
--- a/c/set-protocol/tests/fixtures/plot-v1.json
+++ b/c/set-protocol/tests/fixtures/plot-v1.json
@@ -1,5 +1,5 @@
 {
-  "version": 2,
+  "version": 1,
   "cases": [
     {"name":"zoom_x_anchor","op":0,"input":[0,0,1,1,2,1,0,0,0.25,0.5],"output":[0.125,0,0.5,1]},
     {"name":"zoom_y_anchor","op":0,"input":[0,0,1,1,1,4,0,0,0.5,0.75],"output":[0,0.5625,1,0.25]},
diff --git a/c/set-protocol/tests/test_plot.c b/c/set-protocol/tests/test_plot.c
index 6615548..640621c 100644
--- a/c/set-protocol/tests/test_plot.c
+++ b/c/set-protocol/tests/test_plot.c
@@ -9,7 +9,7 @@ int main(void) {
     const double y[] = {15, 10, 30, 1};
     const double drag[] = {0, 100, 500, -10, 10, 1};
     double back[4], bad[] = {NAN, 10, 8};
-    assert(set_plot_abi_version() == 2);
+    assert(set_plot_abi_version() == 1);
     assert(set_plot_eval(SET_PLOT_TRANSFORM, zoom, 10, output, 4) == 4);
     assert(output[0] == .125 && output[1] == 0 && output[2] == .5 && output[3] == 1);
     assert(output[4] == 12345);
diff --git a/python/protocan/plot.py b/python/protocan/plot.py
index 967638d..26bfeaf 100644
--- a/python/protocan/plot.py
+++ b/python/protocan/plot.py
@@ -37,7 +37,7 @@ class PlotMath:
         self.library = library
         library.set_plot_abi_version.restype = ctypes.c_uint32
         library.set_plot_abi_version.argtypes = []
-        if library.set_plot_abi_version() != 2:
+        if library.set_plot_abi_version() != 1:
             raise RuntimeError("Unsupported plot ABI")
         library.set_plot_eval.argtypes = [ctypes.c_uint32, ctypes.POINTER(ctypes.c_double),
             ctypes.c_size_t, ctypes.POINTER(ctypes.c_double), ctypes.c_size_t]