feat(altair): implement smith-chart-basic (#3872)

## Implementation: `smith-chart-basic` - altair

Implements the **altair** version of `smith-chart-basic`.

**File:** `plots/smith-chart-basic/implementations/altair.py`

**Parent Issue:** #3792

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/21047488931)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/smith-chart-basic/implementations/altair.py

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+""" pyplots.ai
+smith-chart-basic: Smith Chart for RF/Impedance
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 91/100 | Created: 2026-01-15
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Reference impedance
+Z0 = 50  # ohms
+
+# Generate Smith chart grid - constant resistance circles
+theta = np.linspace(0, 2 * np.pi, 100)
+
+resistance_circles = []
+resistance_values = [0, 0.2, 0.5, 1.0, 2.0, 5.0]
+for r in resistance_values:
+    center_x = r / (r + 1)
+    radius = 1 / (r + 1)
+    x = center_x + radius * np.cos(theta)
+    y = radius * np.sin(theta)
+    mask = x**2 + y**2 <= 1.0
+    for i in range(len(x)):
+        if mask[i]:
+            resistance_circles.append({"x": x[i], "y": y[i], "group": f"r_{r}", "idx": i})
+
+resistance_df = pd.DataFrame(resistance_circles)
+
+# Generate constant reactance arcs
+reactance_arcs = []
+reactance_values = [0.2, 0.5, 1.0, 2.0, 5.0]
+arc_theta = np.linspace(-np.pi, np.pi, 100)
+
+for x_val in reactance_values:
+    # Positive reactance (upper half)
+    center_y = 1 / x_val
+    radius = 1 / x_val
+    x = 1 + radius * np.cos(arc_theta)
+    y = center_y + radius * np.sin(arc_theta)
+    mask = (x**2 + y**2 <= 1.0) & (x >= -0.01)
+    for i in range(len(x)):
+        if mask[i]:
+            reactance_arcs.append({"x": x[i], "y": y[i], "group": f"x_pos_{x_val}", "idx": i})
+
+    # Negative reactance (lower half)
+    y_neg = -center_y - radius * np.sin(arc_theta)
+    for i in range(len(x)):
+        if mask[i]:
+            reactance_arcs.append({"x": x[i], "y": y_neg[i], "group": f"x_neg_{x_val}", "idx": i})
+
+# Zero reactance line (horizontal axis)
+x_line = np.linspace(-1, 1, 50)
+for i, xi in enumerate(x_line):
+    reactance_arcs.append({"x": xi, "y": 0, "group": "x_zero", "idx": i})
+
+reactance_df = pd.DataFrame(reactance_arcs)
+
+# Unit circle boundary
+unit_theta = np.linspace(0, 2 * np.pi, 100)
+unit_circle_df = pd.DataFrame({"x": np.cos(unit_theta), "y": np.sin(unit_theta), "idx": range(len(unit_theta))})
+
+# Generate sample impedance data - antenna impedance sweep from 1-6 GHz
+np.random.seed(42)
+n_points = 50
+frequency = np.linspace(1e9, 6e9, n_points)
+
+# Simulate antenna impedance that traces a spiral pattern on Smith chart
+t = np.linspace(0, 2.5 * np.pi, n_points)
+z_real = 50 * (1 - 0.7 * np.exp(-t / 3))
+z_imag = 40 * np.sin(t) * np.exp(-t / 4)
+
+# Normalize impedance and convert to reflection coefficient (gamma)
+z_norm = (z_real + 1j * z_imag) / Z0
+gamma = (z_norm - 1) / (z_norm + 1)
+
+impedance_df = pd.DataFrame(
+    {
+        "x": gamma.real,
+        "y": gamma.imag,
+        "frequency_ghz": frequency / 1e9,
+        "z_real": z_real,
+        "z_imag": z_imag,
+        "idx": range(n_points),
+    }
+)
+
+# Add frequency labels at key points
+label_indices = [0, 12, 24, 36, 49]
+labels_df = impedance_df.iloc[label_indices].copy()
+labels_df["label"] = labels_df["frequency_ghz"].apply(lambda f: f"{f:.1f} GHz")
+
+# Scale domain for consistent axes
+scale_x = alt.Scale(domain=[-1.2, 1.2])
+scale_y = alt.Scale(domain=[-1.2, 1.2])
+
+# Unit circle boundary
+boundary = (
+    alt.Chart(unit_circle_df)
+    .mark_line(color="#306998", strokeWidth=4)
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), order="idx:O")
+)
+
+# Resistance circles
+res_circles = (
+    alt.Chart(resistance_df)
+    .mark_line(strokeWidth=1.5, opacity=0.4, color="#666666")
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), detail="group:N", order="idx:O")
+)
+
+# Reactance arcs
+react_arcs = (
+    alt.Chart(reactance_df)
+    .mark_line(strokeWidth=1.5, opacity=0.4, color="#888888")
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), detail="group:N", order="idx:O")
+)
+
+# Impedance locus curve
+impedance_line = (
+    alt.Chart(impedance_df)
+    .mark_line(strokeWidth=5, color="#FFD43B")
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), order="idx:O")
+)
+
+# Impedance data points
+impedance_points = (
+    alt.Chart(impedance_df)
+    .mark_circle(size=120, color="#FFD43B", stroke="#306998", strokeWidth=1)
+    .encode(
+        x=alt.X("x:Q", scale=scale_x),
+        y=alt.Y("y:Q", scale=scale_y),
+        tooltip=[
+            alt.Tooltip("frequency_ghz:Q", title="Frequency (GHz)", format=".2f"),
+            alt.Tooltip("z_real:Q", title="R (Ω)", format=".1f"),
+            alt.Tooltip("z_imag:Q", title="X (Ω)", format=".1f"),
+        ],
+    )
+)
+
+# Frequency labels
+freq_labels = (
+    alt.Chart(labels_df)
+    .mark_text(fontSize=18, fontWeight="bold", color="#306998", dx=18, dy=-18)
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), text="label:N")
+)
+
+# Center point marker (matched condition Z = Z0)
+center_df = pd.DataFrame({"x": [0], "y": [0]})
+center_point = (
+    alt.Chart(center_df)
+    .mark_point(size=300, shape="cross", color="#306998", strokeWidth=3)
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y))
+)
+
+# Resistance value labels
+r_labels_data = [
+    {"x": 0.0, "y": 0.08, "label": "0"},
+    {"x": 0.17, "y": 0.08, "label": "0.2"},
+    {"x": 0.33, "y": 0.08, "label": "0.5"},
+    {"x": 0.5, "y": 0.08, "label": "1"},
+    {"x": 0.67, "y": 0.08, "label": "2"},
+    {"x": 0.83, "y": 0.08, "label": "5"},
+]
+r_labels_df = pd.DataFrame(r_labels_data)
+
+r_labels = (
+    alt.Chart(r_labels_df)
+    .mark_text(fontSize=14, color="#444444", fontWeight="bold")
+    .encode(x=alt.X("x:Q", scale=scale_x), y=alt.Y("y:Q", scale=scale_y), text="label:N")
+)
+
+# Combine all layers
+chart = (
+    alt.layer(res_circles, react_arcs, boundary, center_point, impedance_line, impedance_points, freq_labels, r_labels)
+    .properties(
+        width=1200,
+        height=1200,
+        title=alt.Title(
+            "smith-chart-basic · altair · pyplots.ai",
+            fontSize=28,
+            anchor="middle",
+            subtitle="Antenna Impedance Sweep (1-6 GHz, Z₀ = 50Ω)",
+            subtitleFontSize=20,
+            subtitleColor="#666666",
+        ),
+    )
+    .configure_view(strokeWidth=0)
+    .configure_axis(grid=False, domain=False, labels=False, ticks=False, title=None)
+)
+
+# Save outputs
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")