diff --git a/src/mcltspice/plotting.py b/src/mcltspice/plotting.py
new file mode 100644
index 0000000..f1c9869
--- /dev/null
+++ b/src/mcltspice/plotting.py
@@ -0,0 +1,176 @@
+"""Waveform-plot construction, separated from file I/O.
+
+build_waveform_svg() turns a parsed RawFile into an SVG string plus metadata.
+It does no filesystem access, so it can be unit-tested with a hand-built
+RawFile -- the plot_waveform MCP tool wraps it with the .raw read and SVG write.
+"""
+
+import numpy as np
+
+from .raw_parser import RawFile
+from .svg_plot import plot_bode, plot_spectrum, plot_timeseries, plot_timeseries_multi
+
+
+def build_waveform_svg(
+    raw_data: RawFile,
+    signal_list: list[str],
+    plot_type: str = "auto",
+    max_points: int = 10000,
+    x_min: float | None = None,
+    x_max: float | None = None,
+    y_min: float | None = None,
+    y_max: float | None = None,
+    width: int = 800,
+    height: int | None = None,
+    title: str | None = None,
+) -> dict:
+    """Render one or more signals from a RawFile to an SVG string.
+
+    Returns {"svg", "plot_type", "signal", "points", "dimensions"} on success,
+    or {"error", ...} if a signal is missing or a multi-signal overlay is
+    requested for a non-time-domain plot. plot_type "auto" picks bode (complex
+    frequency data), spectrum (real frequency data), or time (everything else).
+    """
+    is_multi = len(signal_list) > 1
+
+    # Resolve all signal names (exact, case-insensitive)
+    sig_names = [v.name for v in raw_data.variables]
+    sig_lower = {s.lower(): s for s in sig_names}
+    resolved: list[tuple[str, int]] = []  # (actual_name, index)
+    for s in signal_list:
+        idx = raw_data.resolve_index(s)
+        if idx is None:
+            return {
+                "error": f"Signal '{s}' not found",
+                "available_signals": sig_names,
+            }
+        resolved.append((sig_lower[s.lower()], idx))
+
+    # Get x-axis data (time or frequency)
+    x_var = raw_data.variables[0]
+    x_data = raw_data.data[0]
+    is_freq = x_var.name.lower() == "frequency"
+
+    # For single-signal: check complex data (AC analysis)
+    first_values = raw_data.data[resolved[0][1]]
+    is_complex = bool(np.iscomplexobj(first_values))
+
+    # Determine plot type
+    if plot_type == "auto":
+        if is_freq and is_complex:
+            plot_type = "bode"
+        elif is_freq:
+            plot_type = "spectrum"
+        else:
+            plot_type = "time"
+
+    # Multi-signal only supported for time-domain
+    if is_multi and plot_type != "time":
+        return {
+            "error": "Multi-signal overlay is only supported for time-domain plots.",
+            "plot_type": plot_type,
+            "hint": "Use single signal for bode/spectrum, or set plot_type='time'.",
+        }
+
+    # Clip to X range (before downsampling for efficiency)
+    real_x = np.real(x_data)
+    mask = None
+    if x_min is not None or x_max is not None:
+        mask = np.ones(len(real_x), dtype=bool)
+        if x_min is not None:
+            mask &= real_x >= x_min
+        if x_max is not None:
+            mask &= real_x <= x_max
+        x_data = x_data[mask]
+        real_x = real_x[mask]
+
+    # Downsample (stride-based)
+    step = None
+    if max_points and len(x_data) > max_points:
+        step = max(1, len(x_data) // max_points)
+        x_data = x_data[::step]
+        real_x = real_x[::step]
+
+    # Height default
+    if height is None:
+        height = 500 if plot_type == "bode" else 400
+
+    if is_multi:
+        # Multi-signal time-domain overlay
+        traces: list[tuple[str, np.ndarray]] = []
+        for name, idx in resolved:
+            v = raw_data.data[idx]
+            if mask is not None:
+                v = v[mask]
+            if step is not None:
+                v = v[::step]
+            traces.append((name, np.real(v)))
+
+        if title is None:
+            names_str = ", ".join(n for n, _ in resolved)
+            title = f"Time Domain — {names_str}"
+
+        svg = plot_timeseries_multi(
+            time=real_x, traces=traces,
+            title=title, width=width, height=height,
+            x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
+        )
+        actual_name = ", ".join(n for n, _ in resolved)
+    else:
+        # Single-signal path
+        actual_name, sig_idx = resolved[0]
+        values = raw_data.data[sig_idx]
+        if mask is not None:
+            values = values[mask]
+        if step is not None:
+            values = values[::step]
+
+        if np.iscomplexobj(values):
+            mag_db = 20.0 * np.log10(np.maximum(np.abs(values), 1e-30))
+            phase_deg = np.degrees(np.angle(values))
+        else:
+            mag_db = None
+            phase_deg = None
+
+        # Title default
+        if title is None:
+            if plot_type == "bode":
+                title = f"Bode Plot — {actual_name}"
+            elif plot_type == "spectrum":
+                title = f"Spectrum — {actual_name}"
+            else:
+                title = f"Time Domain — {actual_name}"
+
+        # Generate SVG
+        if plot_type == "bode":
+            if mag_db is None:
+                mag_db = np.real(values)
+                phase_deg = None
+            svg = plot_bode(
+                freq=real_x, mag_db=mag_db, phase_deg=phase_deg,
+                title=title, width=width, height=height,
+                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
+            )
+        elif plot_type == "spectrum":
+            if mag_db is None:
+                mag_db = 20.0 * np.log10(np.maximum(np.abs(np.real(values)), 1e-30))
+            svg = plot_spectrum(
+                freq=real_x, mag_db=mag_db,
+                title=title, width=width, height=height,
+                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
+            )
+        else:  # time
+            svg = plot_timeseries(
+                time=real_x, values=np.real(values),
+                title=title, ylabel=actual_name,
+                width=width, height=height,
+                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
+            )
+
+    return {
+        "svg": svg,
+        "plot_type": plot_type,
+        "signal": actual_name,
+        "points": len(real_x),
+        "dimensions": f"{width}x{height}",
+    }
diff --git a/src/mcltspice/server.py b/src/mcltspice/server.py
index 9e5cc7d..426998d 100644
--- a/src/mcltspice/server.py
+++ b/src/mcltspice/server.py
@@ -49,6 +49,7 @@ from .optimizer import (
     format_engineering,
     optimize_component_values,
 )
+from .plotting import build_waveform_svg
 from .power_analysis import compute_efficiency, compute_power_metrics
 from .raw_parser import parse_raw_file
 from .runner import run_netlist, run_simulation
@@ -59,7 +60,6 @@ from .spicebook import (
     ltspice_to_ngspice,
 )
 from .stability import compute_stability_metrics
-from .svg_plot import plot_bode, plot_spectrum, plot_timeseries, plot_timeseries_multi
 from .templates import (
     ASC_TEMPLATES,
     NETLIST_TEMPLATES,
@@ -1296,158 +1296,36 @@ async def plot_waveform(
 
     # Build signal list (signals overrides signal when set)
     signal_list = signals if signals else [signal]
-    is_multi = len(signal_list) > 1
 
-    # Resolve all signal names (exact, case-insensitive)
-    sig_names = [v.name for v in raw_data.variables]
-    sig_lower = {s.lower(): s for s in sig_names}
-    resolved: list[tuple[str, int]] = []  # (actual_name, index)
-    for s in signal_list:
-        idx = raw_data.resolve_index(s)
-        if idx is None:
-            return {
-                "error": f"Signal '{s}' not found",
-                "available_signals": sig_names,
-            }
-        resolved.append((sig_lower[s.lower()], idx))
-
-    # Get x-axis data (time or frequency)
-    x_var = raw_data.variables[0]
-    x_data = raw_data.data[0]
-    is_freq = x_var.name.lower() == "frequency"
-
-    # For single-signal: check complex data (AC analysis)
-    first_values = raw_data.data[resolved[0][1]]
-    if np.iscomplexobj(first_values):
-        is_complex = True
-    else:
-        is_complex = False
-
-    # Determine plot type
-    if plot_type == "auto":
-        if is_freq and is_complex:
-            plot_type = "bode"
-        elif is_freq:
-            plot_type = "spectrum"
-        else:
-            plot_type = "time"
-
-    # Multi-signal only supported for time-domain
-    if is_multi and plot_type != "time":
-        return {
-            "error": "Multi-signal overlay is only supported for time-domain plots.",
-            "plot_type": plot_type,
-            "hint": "Use single signal for bode/spectrum, or set plot_type='time'.",
-        }
-
-    # Clip to X range (before downsampling for efficiency)
-    real_x = np.real(x_data)
-    mask = None
-    if x_min is not None or x_max is not None:
-        mask = np.ones(len(real_x), dtype=bool)
-        if x_min is not None:
-            mask &= real_x >= x_min
-        if x_max is not None:
-            mask &= real_x <= x_max
-        x_data = x_data[mask]
-        real_x = real_x[mask]
-
-    # Downsample (stride-based, same pattern as get_waveform)
-    step = None
-    if max_points and len(x_data) > max_points:
-        step = max(1, len(x_data) // max_points)
-        x_data = x_data[::step]
-        real_x = real_x[::step]
-
-    # Height default
-    if height is None:
-        height = 500 if plot_type == "bode" else 400
-
-    if is_multi:
-        # Multi-signal time-domain overlay
-        traces: list[tuple[str, np.ndarray]] = []
-        for name, idx in resolved:
-            v = raw_data.data[idx]
-            if mask is not None:
-                v = v[mask]
-            if step is not None:
-                v = v[::step]
-            traces.append((name, np.real(v)))
-
-        if title is None:
-            names_str = ", ".join(n for n, _ in resolved)
-            title = f"Time Domain \u2014 {names_str}"
-
-        svg = plot_timeseries_multi(
-            time=real_x, traces=traces,
-            title=title, width=width, height=height,
-            x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
-        )
-        actual_name = ", ".join(n for n, _ in resolved)
-    else:
-        # Single-signal path (original logic)
-        actual_name, sig_idx = resolved[0]
-        values = raw_data.data[sig_idx]
-        if mask is not None:
-            values = values[mask]
-        if step is not None:
-            values = values[::step]
-
-        if np.iscomplexobj(values):
-            mag_db = 20.0 * np.log10(np.maximum(np.abs(values), 1e-30))
-            phase_deg = np.degrees(np.angle(values))
-        else:
-            mag_db = None
-            phase_deg = None
-
-        # Title default
-        if title is None:
-            if plot_type == "bode":
-                title = f"Bode Plot \u2014 {actual_name}"
-            elif plot_type == "spectrum":
-                title = f"Spectrum \u2014 {actual_name}"
-            else:
-                title = f"Time Domain \u2014 {actual_name}"
-
-        # Generate SVG
-        if plot_type == "bode":
-            if mag_db is None:
-                mag_db = np.real(values)
-                phase_deg = None
-            svg = plot_bode(
-                freq=real_x, mag_db=mag_db, phase_deg=phase_deg,
-                title=title, width=width, height=height,
-                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
-            )
-        elif plot_type == "spectrum":
-            if mag_db is None:
-                mag_db = 20.0 * np.log10(np.maximum(np.abs(np.real(values)), 1e-30))
-            svg = plot_spectrum(
-                freq=real_x, mag_db=mag_db,
-                title=title, width=width, height=height,
-                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
-            )
-        else:  # time
-            svg = plot_timeseries(
-                time=real_x, values=np.real(values),
-                title=title, ylabel=actual_name,
-                width=width, height=height,
-                x_min=x_min, x_max=x_max, y_min=y_min, y_max=y_max,
-            )
+    result = build_waveform_svg(
+        raw_data,
+        signal_list,
+        plot_type=plot_type,
+        max_points=max_points,
+        x_min=x_min,
+        x_max=x_max,
+        y_min=y_min,
+        y_max=y_max,
+        width=width,
+        height=height,
+        title=title,
+    )
+    if "error" in result:
+        return result
 
     # Save SVG
     if output_path is None:
         out = Path(tempfile.mktemp(suffix=".svg", prefix="ltspice_plot_"))
     else:
         out = Path(output_path)
-    out.write_text(svg)
+    out.write_text(result["svg"])
 
     return {
         "svg_path": str(out),
-        "plot_type": plot_type,
-        "signal": actual_name,
-        "points": len(real_x),
-        "dimensions": f"{width}x{height}",
+        "plot_type": result["plot_type"],
+        "signal": result["signal"],
+        "points": result["points"],
+        "dimensions": result["dimensions"],
     }