feat(altair): implement psychrometric-basic (#4901)

## Implementation: `psychrometric-basic` - altair

Implements the **altair** version of `psychrometric-basic`.

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

**Parent Issue:** #4583

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23120046544)*

---------

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/psychrometric-basic/implementations/altair.py

@@ -0,0 +1,363 @@
+""" pyplots.ai
+psychrometric-basic: Psychrometric Chart for HVAC
+Library: altair 6.0.0 | Python 3.14.3
+Quality: 86/100 | Created: 2026-03-15
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Constants
+P_ATM = 101325  # Pa, standard atmospheric pressure
+
+# Precompute saturation pressure over a fine grid (ASHRAE formula, computed once)
+_t_grid = np.linspace(-10, 50, 500)
+_t_grid_k = _t_grid + 273.15
+_p_sat_grid = np.where(
+    _t_grid >= 0,
+    np.exp(
+        -5.8002206e3 / _t_grid_k
+        + 1.3914993
+        - 4.8640239e-2 * _t_grid_k
+        + 4.1764768e-5 * _t_grid_k**2
+        - 1.4452093e-8 * _t_grid_k**3
+        + 6.5459673 * np.log(_t_grid_k)
+    ),
+    np.exp(
+        -5.6745359e3 / _t_grid_k
+        + 6.3925247
+        - 9.677843e-3 * _t_grid_k
+        + 6.2215701e-7 * _t_grid_k**2
+        + 2.0747825e-9 * _t_grid_k**3
+        - 9.484024e-13 * _t_grid_k**4
+        + 4.1635019 * np.log(_t_grid_k)
+    ),
+)
+
+# Interpolation-based saturation pressure lookup (replaces 6x formula repetition)
+p_sat_at = lambda t: np.interp(t, _t_grid, _p_sat_grid)  # noqa: E731
+
+# Data - Generate psychrometric curves
+t_range = np.linspace(-10, 50, 200)
+p_sat = p_sat_at(t_range)
+
+# Relative humidity curves (10% to 100%)
+rh_curves = []
+for rh_pct in range(10, 110, 10):
+    rh_frac = rh_pct / 100
+    p_w = rh_frac * p_sat
+    w_vals = 0.621945 * p_w / (P_ATM - p_w) * 1000  # g/kg
+    w_vals = np.clip(w_vals, 0, 30)
+    for t, w in zip(t_range, w_vals, strict=True):
+        if 0 < w <= 30:
+            rh_curves.append({"t_db": float(t), "w": float(w), "rh": f"{rh_pct}%"})
+
+rh_df = pd.DataFrame(rh_curves)
+
+# Wet-bulb temperature lines
+wb_lines = []
+for t_wb_val in range(0, 36, 5):
+    p_sat_wb = float(p_sat_at(t_wb_val))
+    w_s_wb = 0.621945 * p_sat_wb / (P_ATM - p_sat_wb)
+    for t_db in np.linspace(max(-10, t_wb_val), 50, 80):
+        w = (2501 * w_s_wb - 1.006 * (t_db - t_wb_val)) / (2501 + 1.86 * t_db - 4.186 * t_wb_val)
+        w_gkg = w * 1000
+        if 0 <= w_gkg <= 30:
+            wb_lines.append({"t_db": float(t_db), "w": float(w_gkg), "wb": f"{t_wb_val}°C"})
+
+wb_df = pd.DataFrame(wb_lines)
+
+# Enthalpy lines (h = 1.006*t + w*(2501 + 1.86*t), solve for w)
+enthalpy_lines = []
+for h_val in range(10, 120, 10):
+    for t_db in np.linspace(-10, 50, 80):
+        w_gkg = (h_val - 1.006 * t_db) / (2.501 + 0.00186 * t_db)
+        if 0 <= w_gkg <= 30:
+            enthalpy_lines.append({"t_db": float(t_db), "w": float(w_gkg), "h": f"{h_val} kJ/kg"})
+
+enthalpy_df = pd.DataFrame(enthalpy_lines)
+
+# Specific volume lines (v = 0.287042*T_k*(1+1.6078*w)/P, solve for w)
+vol_lines = []
+for v_val in [0.75, 0.80, 0.85, 0.90, 0.95]:
+    for t_db in np.linspace(-10, 50, 80):
+        w = (v_val * P_ATM / 1000 / (0.287042 * (t_db + 273.15)) - 1) / 1.6078
+        w_gkg = w * 1000
+        if 0 <= w_gkg <= 30:
+            vol_lines.append({"t_db": float(t_db), "w": float(w_gkg), "v": f"{v_val} m³/kg"})
+
+vol_df = pd.DataFrame(vol_lines)
+
+# Comfort zone (20-26°C, 30-60% RH)
+comfort_temps = np.linspace(20, 26, 30)
+comfort_psat = p_sat_at(comfort_temps)
+comfort_w_lo = 0.621945 * 0.30 * comfort_psat / (P_ATM - 0.30 * comfort_psat) * 1000
+comfort_w_hi = 0.621945 * 0.60 * comfort_psat / (P_ATM - 0.60 * comfort_psat) * 1000
+comfort_df = pd.DataFrame({"t_db": comfort_temps, "w": comfort_w_lo, "w2": comfort_w_hi})
+
+# HVAC process path: cooling and dehumidification (35°C/50%RH -> 13°C/sat)
+t1, t2 = 35.0, 13.0
+p_sat_t1, p_sat_t2 = float(p_sat_at(t1)), float(p_sat_at(t2))
+w1 = 0.621945 * 0.50 * p_sat_t1 / (P_ATM - 0.50 * p_sat_t1) * 1000
+w2 = 0.621945 * 1.00 * p_sat_t2 / (P_ATM - 1.00 * p_sat_t2) * 1000
+
+process_points = pd.DataFrame(
+    {
+        "t_db": [t1, t2],
+        "w": [float(w1), float(w2)],
+        "label": ["Outdoor Air (35°C, 50% RH)", "Supply Air (13°C, 100% RH)"],
+        "rh_pct": ["50%", "100%"],
+        "order": [0, 1],
+    }
+)
+
+# RH label positions (staggered to avoid overlap)
+rh_labels = []
+for rh_pct in range(10, 110, 10):
+    rh_frac = rh_pct / 100
+    # Stagger label temperatures to reduce convergence overlap
+    if rh_pct == 100:
+        t_label = 33
+    elif rh_pct >= 80:
+        t_label = 36
+    elif rh_pct >= 60:
+        t_label = 40
+    elif rh_pct >= 40:
+        t_label = 44
+    else:
+        t_label = 47
+    p_sat_label = float(p_sat_at(t_label))
+    w_label = 0.621945 * rh_frac * p_sat_label / (P_ATM - rh_frac * p_sat_label) * 1000
+    if w_label <= 30:
+        rh_labels.append({"t_db": float(t_label), "w": float(w_label), "label": f"{rh_pct}%"})
+
+rh_label_df = pd.DataFrame(rh_labels)
+
+# Wet-bulb labels (offset from saturation curve to avoid overlap with enthalpy labels)
+wb_labels_data = []
+for t_wb_val in range(0, 36, 5):
+    p_sat_wb = float(p_sat_at(t_wb_val))
+    w_wb_label = 0.621945 * p_sat_wb / (P_ATM - p_sat_wb) * 1000
+    if w_wb_label <= 28:
+        wb_labels_data.append({"t_db": float(t_wb_val) + 1.5, "w": float(w_wb_label) + 0.8, "label": f"{t_wb_val}°C"})
+
+wb_label_df = pd.DataFrame(wb_labels_data)
+
+# Enthalpy labels (along left edge, skip values that would overlap with wet-bulb labels)
+enthalpy_labels = []
+for h_val in range(20, 120, 20):
+    w_at_left = (h_val - 1.006 * (-10)) / (2.501 + 0.00186 * (-10))
+    if 0 <= w_at_left <= 30:
+        enthalpy_labels.append({"t_db": -9.5, "w": float(w_at_left), "label": f"{h_val} kJ/kg"})
+    else:
+        t_at_top = (h_val - 2.501 * 30) / (1.006 + 0.00186 * 30)
+        if -10 <= t_at_top <= 50:
+            enthalpy_labels.append({"t_db": float(t_at_top), "w": 30.0, "label": f"{h_val} kJ/kg"})
+
+enthalpy_label_df = pd.DataFrame(enthalpy_labels)
+
+# Volume labels (along bottom-right)
+vol_labels = []
+for v_val in [0.75, 0.80, 0.85, 0.90, 0.95]:
+    w_at_bot = (v_val * P_ATM / 1000 / (0.287042 * (45 + 273.15)) - 1) / 1.6078 * 1000
+    if 0 <= w_at_bot <= 30:
+        vol_labels.append({"t_db": 45.0, "w": float(w_at_bot), "label": f"{v_val} m³/kg"})
+
+vol_label_df = pd.DataFrame(vol_labels)
+
+# Colorblind-safe palette: blue (RH), orange (wet-bulb), teal (enthalpy), purple (volume)
+CLR_RH = "#306998"
+CLR_WB = "#d97b0e"
+CLR_ENTHALPY = "#17becf"
+CLR_VOL = "#9467bd"
+CLR_COMFORT = "#2ecc71"
+CLR_PROCESS = "#c0392b"
+CLR_BG = "#fafbfc"
+
+# Plot
+x_scale = alt.Scale(domain=[-10, 50])
+y_scale = alt.Scale(domain=[0, 30])
+
+# Saturation curve (100% RH) - visually prominent
+sat_df = rh_df[rh_df["rh"] == "100%"]
+saturation = (
+    alt.Chart(sat_df)
+    .mark_line(strokeWidth=3.5, color=CLR_RH)
+    .encode(
+        x=alt.X("t_db:Q", scale=x_scale, title="Dry-Bulb Temperature (°C)"),
+        y=alt.Y("w:Q", scale=y_scale, title="Humidity Ratio (g/kg)"),
+    )
+)
+
+# Other RH curves with tooltips
+other_rh_df = rh_df[rh_df["rh"] != "100%"]
+rh_chart = (
+    alt.Chart(other_rh_df)
+    .mark_line(strokeWidth=1.5, opacity=0.55)
+    .encode(
+        x=alt.X("t_db:Q", scale=x_scale),
+        y=alt.Y("w:Q", scale=y_scale),
+        color=alt.Color("rh:N", scale=alt.Scale(scheme="blues"), legend=None),
+        detail="rh:N",
+        tooltip=[
+            alt.Tooltip("t_db:Q", title="Dry-Bulb (°C)", format=".1f"),
+            alt.Tooltip("w:Q", title="Humidity (g/kg)", format=".1f"),
+            alt.Tooltip("rh:N", title="Relative Humidity"),
+        ],
+    )
+)
+
+# RH labels
+rh_text = (
+    alt.Chart(rh_label_df)
+    .mark_text(fontSize=13, color="#4a7fb5", fontWeight="bold")
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Wet-bulb lines (orange, colorblind-safe)
+wb_chart = (
+    alt.Chart(wb_df)
+    .mark_line(strokeWidth=1, strokeDash=[6, 4], opacity=0.5, color=CLR_WB)
+    .encode(
+        x=alt.X("t_db:Q", scale=x_scale),
+        y=alt.Y("w:Q", scale=y_scale),
+        detail="wb:N",
+        tooltip=[
+            alt.Tooltip("t_db:Q", title="Dry-Bulb (°C)", format=".1f"),
+            alt.Tooltip("w:Q", title="Humidity (g/kg)", format=".1f"),
+            alt.Tooltip("wb:N", title="Wet-Bulb Temp"),
+        ],
+    )
+)
+
+# Wet-bulb labels (offset to avoid overlap)
+wb_text = (
+    alt.Chart(wb_label_df)
+    .mark_text(fontSize=12, color=CLR_WB, align="left", dx=2, dy=-6, fontWeight="bold")
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Enthalpy lines (teal, colorblind-safe)
+enthalpy_chart = (
+    alt.Chart(enthalpy_df)
+    .mark_line(strokeWidth=1, strokeDash=[4, 6], opacity=0.45, color=CLR_ENTHALPY)
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), detail="h:N")
+)
+
+# Enthalpy labels
+enthalpy_text = (
+    alt.Chart(enthalpy_label_df)
+    .mark_text(fontSize=11, color=CLR_ENTHALPY, align="left", dx=2, dy=-4, fontWeight="bold")
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Specific volume lines
+vol_chart = (
+    alt.Chart(vol_df)
+    .mark_line(strokeWidth=1, strokeDash=[2, 4], opacity=0.4, color=CLR_VOL)
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), detail="v:N")
+)
+
+# Volume labels
+vol_text = (
+    alt.Chart(vol_label_df)
+    .mark_text(fontSize=11, color=CLR_VOL, align="left", dx=3, dy=-5, fontWeight="bold")
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Comfort zone shaded area
+comfort = (
+    alt.Chart(comfort_df)
+    .mark_area(opacity=0.12, color=CLR_COMFORT)
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), y2="w2:Q")
+)
+
+comfort_label = (
+    alt.Chart(pd.DataFrame({"t_db": [23.0], "w": [10.5], "label": ["Comfort Zone"]}))
+    .mark_text(fontSize=14, color="#27ae60", fontWeight="bold", fontStyle="italic")
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Interactive selection for HVAC process points
+point_selection = alt.selection_point(on="pointerover", nearest=True, fields=["t_db"])
+
+# HVAC process path with tooltips
+process_line = (
+    alt.Chart(process_points)
+    .mark_line(strokeWidth=3.5, color=CLR_PROCESS, point=alt.OverlayMarkDef(size=140, filled=True, color=CLR_PROCESS))
+    .encode(
+        x=alt.X("t_db:Q", scale=x_scale),
+        y=alt.Y("w:Q", scale=y_scale),
+        order="order:Q",
+        tooltip=[
+            alt.Tooltip("label:N", title="State Point"),
+            alt.Tooltip("t_db:Q", title="Dry-Bulb (°C)", format=".1f"),
+            alt.Tooltip("w:Q", title="Humidity (g/kg)", format=".1f"),
+            alt.Tooltip("rh_pct:N", title="RH"),
+        ],
+    )
+    .add_params(point_selection)
+)
+
+# Process labels (adjusted positions to avoid overlap)
+outdoor_label = (
+    alt.Chart(process_points[process_points["order"] == 0])
+    .mark_text(fontSize=12, fontWeight="bold", color=CLR_PROCESS, align="right", dx=-12, dy=-14)
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+supply_label = (
+    alt.Chart(process_points[process_points["order"] == 1])
+    .mark_text(fontSize=12, fontWeight="bold", color=CLR_PROCESS, align="left", dx=12, dy=14)
+    .encode(x=alt.X("t_db:Q", scale=x_scale), y=alt.Y("w:Q", scale=y_scale), text="label:N")
+)
+
+# Layer all elements
+chart = (
+    alt.layer(
+        comfort,
+        rh_chart,
+        saturation,
+        wb_chart,
+        enthalpy_chart,
+        vol_chart,
+        rh_text,
+        wb_text,
+        enthalpy_text,
+        vol_text,
+        comfort_label,
+        process_line,
+        outdoor_label,
+        supply_label,
+    )
+    .properties(
+        width=1600,
+        height=900,
+        title=alt.Title(
+            text="psychrometric-basic · altair · pyplots.ai",
+            fontSize=28,
+            anchor="middle",
+            subtitle="Standard Atmosphere (101.325 kPa) · HVAC Air Properties",
+            subtitleFontSize=18,
+            subtitleColor="#888888",
+            offset=12,
+        ),
+    )
+    .configure_axis(
+        labelFontSize=18,
+        titleFontSize=22,
+        titleColor="#444444",
+        labelColor="#555555",
+        grid=True,
+        gridOpacity=0.12,
+        gridColor="#cccccc",
+        domainColor="#999999",
+    )
+    .configure_view(strokeWidth=0, fill=CLR_BG)
+)
+
+# Save
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")