diff --git a/python/xy/_native.py b/python/xy/_native.py
index ca9cfd7..6296481 100644
--- a/python/xy/_native.py
+++ b/python/xy/_native.py
@@ -24,7 +24,7 @@
 
 from .config import MAX_CONTOUR_WORK, MAX_SCREEN_DIM
 
-ABI_VERSION = 35
+ABI_VERSION = 36
 
 # Rust reports invalid arguments (and, via the ffi_guard panic shield, any
 # internal panic) by returning `usize::MAX` from size-returning entry points.
@@ -166,6 +166,25 @@ def _load() -> ctypes.CDLL:
         ctypes.c_size_t,
         ctypes.c_void_p,
     ]
+    lib.xy_svg_poly_path.restype = ctypes.c_size_t
+    lib.xy_svg_poly_path.argtypes = [
+        ctypes.c_void_p,
+        ctypes.c_void_p,
+        ctypes.c_size_t,
+        ctypes.c_void_p,
+        ctypes.c_size_t,
+    ]
+    lib.xy_m4_points.restype = ctypes.c_size_t
+    lib.xy_m4_points.argtypes = [
+        ctypes.c_void_p,
+        ctypes.c_void_p,
+        ctypes.c_size_t,
+        ctypes.c_double,
+        ctypes.c_double,
+        ctypes.c_size_t,
+        ctypes.c_void_p,
+        ctypes.c_void_p,
+    ]
     lib.xy_stacked_bounds.restype = ctypes.c_int32
     lib.xy_stacked_bounds.argtypes = [
         ctypes.c_void_p,
@@ -1779,6 +1798,60 @@ def m4_indices(
     return out[:written].copy()
 
 
+def m4_points(
+    x: npt.NDArray[np.float64],
+    y: npt.NDArray[np.float64],
+    x0: float,
+    x1: float,
+    n_buckets: int,
+) -> tuple[npt.NDArray[np.float64], npt.NDArray[np.float64]]:
+    """M4-decimate an x/y pair without materializing gather indices in Python."""
+    n_buckets = _bounded_positive_int(n_buckets, "n_buckets")
+    x0, x1 = _finite_increasing(x0, x1, "x range")
+    x = _as_f64(x, "x")
+    y = _as_f64(y, "y")
+    if len(x) != len(y):
+        raise ValueError("x and y must have equal length")
+    if len(x) == 0:
+        empty = np.empty(0, dtype=np.float64)
+        return empty, empty.copy()
+    out_x = np.empty(n_buckets * 4, dtype=np.float64)
+    out_y = np.empty(n_buckets * 4, dtype=np.float64)
+    written = _lib.xy_m4_points(
+        _ptr_f64(x),
+        _ptr_f64(y),
+        len(x),
+        x0,
+        x1,
+        n_buckets,
+        _ptr_f64(out_x),
+        _ptr_f64(out_y),
+    )
+    if written == _USIZE_MAX:
+        raise ValueError("invalid m4 arguments")
+    return out_x[:written], out_y[:written]
+
+
+def svg_poly_path(x: npt.ArrayLike, y: npt.ArrayLike) -> str:
+    """Serialize parallel screen coordinates as SVG path data in Rust."""
+    xa = np.ascontiguousarray(x, dtype=np.float64).reshape(-1)
+    ya = np.ascontiguousarray(y, dtype=np.float64).reshape(-1)
+    if len(xa) != len(ya) or len(xa) == 0:
+        raise ValueError("x and y must be non-empty and have equal length")
+    # Normal chart coordinates fit comfortably. The ABI returns the exact
+    # requirement without writing when an adversarial fixed-point value needs
+    # more room, so the uncommon retry remains allocation-safe.
+    capacity = max(64, len(xa) * 32)
+    while True:
+        out = ctypes.create_string_buffer(capacity)
+        written = _lib.xy_svg_poly_path(_ptr_f64(xa), _ptr_f64(ya), len(xa), out, capacity)
+        if written == _USIZE_MAX:
+            raise ValueError("invalid SVG polyline coordinates")
+        if written <= capacity:
+            return out.raw[:written].decode("ascii")
+        capacity = written
+
+
 def marching_squares(
     z: npt.NDArray[np.float64],
     x_coords: npt.NDArray[np.float64],
diff --git a/python/xy/_payload.py b/python/xy/_payload.py
index c48111f..739c24b 100644
--- a/python/xy/_payload.py
+++ b/python/xy/_payload.py
@@ -10,7 +10,7 @@
 
 import numpy as np
 
-from . import channels, kernels, lod
+from . import _native, channels, kernels, lod
 from ._trace import Trace
 from .columns import Column
 from .config import (
@@ -347,6 +347,11 @@ def _m4_decimate(
         Returns `(tier, arrays)` — shared by the line and area emitters."""
         if t.n_points <= DECIMATION_THRESHOLD:
             return "direct", arrays
+        if len(arrays) == 2:
+            selected = _native.m4_points(
+                arrays[0], arrays[1], xr[0], xr[1] + np.finfo(np.float64).eps, px_width
+            )
+            return "decimated", selected
         idx = kernels.m4_indices(
             arrays[0], arrays[1], xr[0], xr[1] + np.finfo(np.float64).eps, px_width
         )
diff --git a/python/xy/_svg.py b/python/xy/_svg.py
index 13ddd5e..0e28fb4 100644
--- a/python/xy/_svg.py
+++ b/python/xy/_svg.py
@@ -28,7 +28,7 @@
 
 import numpy as np
 
-from . import _png
+from . import _native, _png
 from ._arrowgeom import arrow_shapes as _arrow_shapes
 from .config import DEFAULT_PALETTE
 
@@ -836,9 +836,7 @@ def _rounded_rect_path(
 
 
 def _poly_path(px: np.ndarray, py: np.ndarray) -> str:
-    parts = [f"M {_num(px[0])} {_num(py[0])}"]
-    parts.extend(f"L {_num(px[i])} {_num(py[i])}" for i in range(1, len(px)))
-    return " ".join(parts)
+    return _native.svg_poly_path(px, py)
 
 
 def _curve_path(xv: np.ndarray, yv: np.ndarray, sx: _Scale, sy: _Scale, smooth: bool) -> str:
diff --git a/src/lib.rs b/src/lib.rs
index 8026f5c..dc8cae6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,6 +24,7 @@ mod font;
 pub mod kernels;
 pub mod raster;
 mod simd;
+pub mod svg;
 pub mod tiles;
 
 use kernels::ZoneMap;
@@ -79,7 +80,7 @@ unsafe fn borrowed_byte_spans<'a>(
 /// ABI version — bumped on any signature change. The Python wrapper checks this
 /// at load time and refuses a mismatched library loudly (§33 comm-versioning
 /// rule, applied to the in-process boundary).
-pub const ABI_VERSION: u32 = 35;
+pub const ABI_VERSION: u32 = 36;
 const FACTORIZE_CAPACITY_EXCEEDED: usize = usize::MAX - 1;
 
 #[no_mangle]
@@ -87,6 +88,40 @@ pub extern "C" fn xy_abi_version() -> u32 {
     ABI_VERSION
 }
 
+/// Serialize parallel f64 screen coordinates into SVG polyline path data.
+/// Returns the required byte count, or `usize::MAX` for invalid inputs. When
+/// `out_cap` is too small no bytes are written, allowing callers to retry.
+///
+/// # Safety
+/// `x` and `y` must point to `len` readable f64s. When `out_cap` is sufficient,
+/// `out` must point to `out_cap` writable bytes.
+#[no_mangle]
+pub unsafe extern "C" fn xy_svg_poly_path(
+    x: *const f64,
+    y: *const f64,
+    len: usize,
+    out: *mut u8,
+    out_cap: usize,
+) -> usize {
+    if len == 0 || x.is_null() || y.is_null() {
+        return usize::MAX;
+    }
+    let x = std::slice::from_raw_parts(x, len);
+    let y = std::slice::from_raw_parts(y, len);
+    let Some(path) = ffi_guard(None, || svg::poly_path(x, y)) else {
+        return usize::MAX;
+    };
+    let required = path.len();
+    if out_cap < required {
+        return required;
+    }
+    if out.is_null() {
+        return usize::MAX;
+    }
+    std::slice::from_raw_parts_mut(out, out_cap)[..required].copy_from_slice(path.as_bytes());
+    required
+}
+
 /// Factor `len` fixed-width records into first-seen u32 codes. Returns the
 /// number of unique records or `usize::MAX` on invalid pointers/dimensions.
 ///
@@ -555,6 +590,56 @@ pub unsafe extern "C" fn xy_m4_indices(
     idx.len()
 }
 
+/// Fused M4 decimation for a parallel x/y pair. Unlike `xy_m4_indices`, this
+/// writes the selected values directly and avoids returning an index array for
+/// Python to gather through NumPy. SVG and PNG payload construction both use
+/// this entry point, so their common reduction path stays native.
+///
+/// Returns the number of values written, or `usize::MAX` on invalid input.
+/// `out_x` and `out_y` must each hold `4 * n_buckets` f64 values.
+///
+/// # Safety
+/// `x` and `y` must point to `len` readable f64s; both output pointers must
+/// address the documented writable capacity.
+#[no_mangle]
+pub unsafe extern "C" fn xy_m4_points(
+    x: *const f64,
+    y: *const f64,
+    len: usize,
+    x0: f64,
+    x1: f64,
+    n_buckets: usize,
+    out_x: *mut f64,
+    out_y: *mut f64,
+) -> usize {
+    if n_buckets == 0 || !finite_gt(x0, x1) || len > u32::MAX as usize {
+        return usize::MAX;
+    }
+    let out_len = match n_buckets.checked_mul(4) {
+        Some(n) => n,
+        None => return usize::MAX,
+    };
+    if len == 0 {
+        return 0;
+    }
+    if x.is_null() || y.is_null() || out_x.is_null() || out_y.is_null() {
+        return usize::MAX;
+    }
+    let x = std::slice::from_raw_parts(x, len);
+    let y = std::slice::from_raw_parts(y, len);
+    let Some(idx) = ffi_guard(None, || Some(kernels::m4_indices(x, y, x0, x1, n_buckets))) else {
+        return usize::MAX;
+    };
+    let out_x = std::slice::from_raw_parts_mut(out_x, out_len);
+    let out_y = std::slice::from_raw_parts_mut(out_y, out_len);
+    for (dst, &source) in idx.iter().enumerate() {
+        let source = source as usize;
+        out_x[dst] = x[source];
+        out_y[dst] = y[source];
+    }
+    idx.len()
+}
+
 /// Native stacked-series layout. `values`, `out_lower`, and `out_upper` are
 /// row-major `rows * cols` f64 buffers. `baseline` uses the generic engine
 /// layout ids documented by `kernels::stacked_bounds_into`.
diff --git a/src/svg.rs b/src/svg.rs
new file mode 100644
index 0000000..30cddb4
--- /dev/null
+++ b/src/svg.rs
@@ -0,0 +1,58 @@
+//! Focused native SVG serialization helpers.
+//!
+//! SVG and PNG share screen-space geometry.  Keeping the high-cardinality
+//! coordinate serialization here avoids creating one Python string per point;
+//! broader scene construction can migrate behind the same private boundary.
+
+use std::fmt::Write;
+
+#[inline]
+fn push_num(out: &mut String, value: f64) {
+    let start = out.len();
+    write!(out, "{value:.2}").expect("writing to String cannot fail");
+    while out.as_bytes().last() == Some(&b'0') {
+        out.pop();
+    }
+    if out.as_bytes().last() == Some(&b'.') {
+        out.pop();
+    }
+    // Match Python's fixed-point formatter for negative values rounding to 0.
+    if &out[start..] == "-0" {
+        out.truncate(start);
+        out.push_str("-0");
+    }
+}
+
+/// Serialize parallel screen-space coordinates as an SVG path data string.
+pub fn poly_path(x: &[f64], y: &[f64]) -> Option<String> {
+    if x.len() != y.len() || x.is_empty() || x.iter().chain(y).any(|v| !v.is_finite()) {
+        return None;
+    }
+    let mut out = String::with_capacity(x.len().saturating_mul(24));
+    for (i, (&x, &y)) in x.iter().zip(y).enumerate() {
+        if i != 0 {
+            out.push(' ');
+        }
+        out.push(if i == 0 { 'M' } else { 'L' });
+        out.push(' ');
+        push_num(&mut out, x);
+        out.push(' ');
+        push_num(&mut out, y);
+    }
+    Some(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn poly_path_matches_public_svg_number_format() {
+        assert_eq!(
+            poly_path(&[1.0, 2.345, -0.001], &[4.5, 6.789, -0.0]).as_deref(),
+            Some("M 1 4.5 L 2.35 6.79 L -0 -0")
+        );
+        assert!(poly_path(&[], &[]).is_none());
+        assert!(poly_path(&[1.0], &[f64::NAN]).is_none());
+    }
+}
diff --git a/tests/test_kernels.py b/tests/test_kernels.py
index 9289b85..83200e7 100644
--- a/tests/test_kernels.py
+++ b/tests/test_kernels.py
@@ -1190,3 +1190,26 @@ def test_density_rgba_validates_shape_and_domain():
         k.density_rgba(np.zeros(4, dtype=np.uint8), 2, 2, -1.0, stops, 1.0)
     with pytest.raises(ValueError, match="opacity"):
         k.density_rgba(np.zeros(4, dtype=np.uint8), 2, 2, 1.0, stops, 1.1)
+
+
+def test_native_svg_poly_path_format_and_validation():
+    from xy import _native
+
+    assert _native.svg_poly_path([1.0, 2.345, -0.001], [4.5, 6.789, -0.0]) == (
+        "M 1 4.5 L 2.35 6.79 L -0 -0"
+    )
+    with pytest.raises(ValueError, match="non-empty"):
+        _native.svg_poly_path([], [])
+    with pytest.raises(ValueError, match="coordinates"):
+        _native.svg_poly_path([1.0], [np.nan])
+
+
+def test_native_m4_points_matches_index_gather():
+    from xy import _native
+
+    x = np.arange(10_000, dtype=np.float64)
+    y = np.sin(x * 0.01)
+    idx = _native.m4_indices(x, y, 500.0, 9_500.0, 128)
+    selected_x, selected_y = _native.m4_points(x, y, 500.0, 9_500.0, 128)
+    np.testing.assert_array_equal(selected_x, x[idx])
+    np.testing.assert_array_equal(selected_y, y[idx])