feat(pygal): implement psychrometric-basic (#4900)

## Implementation: `psychrometric-basic` - pygal

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

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

**Parent Issue:** #4583

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/23120046646)*

---------

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/pygal.py

@@ -0,0 +1,386 @@
+""" pyplots.ai
+psychrometric-basic: Psychrometric Chart for HVAC
+Library: pygal 3.1.0 | Python 3.14.3
+Quality: 78/100 | Created: 2026-03-15
+"""
+
+import math
+
+import cairosvg
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Constants
+P_ATM = 101325.0  # Pa (standard atmosphere)
+
+
+def sat_pressure(t_celsius):
+    """ASHRAE saturation vapor pressure (Pa) from dry-bulb temperature."""
+    tk = t_celsius + 273.15
+    if t_celsius >= 0:
+        ln_p = (
+            -5.8002206e3 / tk
+            + 1.3914993
+            - 4.8640239e-2 * tk
+            + 4.1764768e-5 * tk**2
+            - 1.4452093e-8 * tk**3
+            + 6.5459673 * math.log(tk)
+        )
+    else:
+        ln_p = (
+            -5.6745359e3 / tk
+            + 6.3925247
+            - 9.6778430e-3 * tk
+            + 6.2215701e-7 * tk**2
+            + 2.0747825e-9 * tk**3
+            - 9.4840240e-13 * tk**4
+            + 4.1635019 * math.log(tk)
+        )
+    return math.exp(ln_p)
+
+
+def humidity_ratio(t_celsius, rh):
+    """Humidity ratio (g/kg) from temperature and relative humidity (0-1)."""
+    pw = rh * sat_pressure(t_celsius)
+    return 0.62198 * pw / (P_ATM - pw) * 1000
+
+
+def specific_volume_w(t_celsius, sv_target):
+    """Solve analytically for humidity ratio (kg/kg) given temperature and target specific volume."""
+    tk = t_celsius + 273.15
+    return (sv_target * P_ATM / 1000 / (0.287042 * tk) - 1) / 1.6078
+
+
+# Data — precompute all psychrometric curves
+t_range = np.linspace(-10, 50, 250)
+pws_arr = np.array([sat_pressure(float(t)) for t in t_range])
+
+rh_levels = [1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1]
+rh_curves = {}
+for rh in rh_levels:
+    pts = []
+    for i, t in enumerate(t_range):
+        pw = rh * pws_arr[i]
+        w_gkg = 0.62198 * pw / (P_ATM - pw) * 1000
+        if 0 <= w_gkg <= 30:
+            pts.append((round(float(t), 2), round(w_gkg, 3)))
+    rh_curves[rh] = pts
+
+# Wet-bulb temperature lines (Stull 2011 approximation)
+wb_temps = [0, 5, 10, 15, 20, 25, 30]
+wb_lines = {}
+for tw_target in wb_temps:
+    pts = []
+    for t in np.linspace(max(-10, tw_target), min(tw_target + 30, 50), 100):
+        t_f = float(t)
+        for rh_pct in np.linspace(1, 100, 500):
+            tw = (
+                t_f * math.atan(0.151977 * math.sqrt(rh_pct + 8.313659))
+                + math.atan(t_f + rh_pct)
+                - math.atan(rh_pct - 1.676331)
+                + 0.00391838 * rh_pct**1.5 * math.atan(0.023101 * rh_pct)
+                - 4.686035
+            )
+            if abs(tw - tw_target) < 0.25:
+                w_gkg = humidity_ratio(t_f, rh_pct / 100)
+                w_sat = humidity_ratio(t_f, 1.0)
+                if 0 <= w_gkg <= min(30, w_sat + 0.2):
+                    pts.append((round(t_f, 2), round(w_gkg, 3)))
+                break
+    if len(pts) > 2:
+        pts.sort(key=lambda p: p[0])
+        step = max(1, len(pts) // 30)
+        wb_lines[tw_target] = pts[::step]
+
+# Specific volume lines — analytical solution
+sv_values = [0.80, 0.84, 0.88, 0.92, 0.96]
+sv_lines = {}
+for sv_target in sv_values:
+    pts = []
+    for t in np.linspace(-10, 50, 200):
+        t_f = float(t)
+        w_kgkg = specific_volume_w(t_f, sv_target)
+        w_gkg = w_kgkg * 1000
+        w_sat = humidity_ratio(t_f, 1.0)
+        if 0 <= w_gkg <= min(30, w_sat + 0.2):
+            pts.append((round(t_f, 2), round(w_gkg, 3)))
+    if len(pts) > 2:
+        pts.sort(key=lambda p: p[0])
+        step = max(1, len(pts) // 25)
+        sv_lines[sv_target] = pts[::step]
+
+# Enthalpy lines (h = 1.006*t + w*(2501 + 1.86*t), w in kg/kg)
+enthalpy_values = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
+enthalpy_lines = {}
+for h_target in enthalpy_values:
+    pts = []
+    for t in np.linspace(-10, 50, 200):
+        t_f = float(t)
+        w_kgkg = (h_target - 1.006 * t_f) / (2501 + 1.86 * t_f)
+        w_gkg = w_kgkg * 1000
+        w_sat = humidity_ratio(t_f, 1.0)
+        if 0 <= w_gkg <= min(30, w_sat + 0.2):
+            pts.append((round(t_f, 2), round(w_gkg, 3)))
+    if len(pts) > 2:
+        pts.sort(key=lambda p: p[0])
+        step = max(1, len(pts) // 25)
+        enthalpy_lines[h_target] = pts[::step]
+
+# Comfort zone polygon (20-26°C, 30-60% RH)
+comfort_t = np.linspace(20, 26, 40)
+comfort_bottom = [humidity_ratio(float(t), 0.30) for t in comfort_t]
+comfort_top = [humidity_ratio(float(t), 0.60) for t in comfort_t]
+comfort_pts = [(round(float(t), 2), round(w, 3)) for t, w in zip(comfort_t, comfort_bottom, strict=True)] + [
+    (round(float(t), 2), round(w, 3)) for t, w in zip(reversed(comfort_t), reversed(comfort_top), strict=False)
+]
+comfort_pts.append(comfort_pts[0])
+
+# HVAC process: cooling and dehumidification (35°C/60%RH → 24°C/50%RH)
+hvac_states = [(35.0, round(humidity_ratio(35, 0.60), 3)), (24.0, round(humidity_ratio(24, 0.50), 3))]
+
+# Color palette — distinct families per property type
+blue_rh = ["#0d3b66", "#1a4d80", "#276099", "#3572b0", "#4384c4", "#5195d4", "#5fa5e0", "#6db5ea", "#7bc4f2", "#89d1f8"]
+orange_wb = "#d4792a"
+purple_sv = "#6a4c93"
+teal_enth = "#2a8a8a"
+
+palette = tuple(blue_rh + [orange_wb] * 7 + [purple_sv] * 5 + [teal_enth] * 10 + ["#4caf6e", "#c62828"])
+
+# Style
+custom_style = Style(
+    background="#ffffff",
+    plot_background="#fafbfc",
+    foreground="#2d2d2d",
+    foreground_strong="#111111",
+    foreground_subtle="#e8ebef",
+    opacity=".85",
+    opacity_hover="1",
+    colors=palette,
+    title_font_size=72,
+    label_font_size=42,
+    major_label_font_size=46,
+    legend_font_size=34,
+    value_font_size=34,
+    stroke_width=2.5,
+    title_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    label_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    major_label_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    legend_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    value_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    tooltip_font_size=24,
+    tooltip_font_family="Trebuchet MS, Helvetica Neue, sans-serif",
+    transition="150ms ease-in",
+    value_colors=(),
+    guide_stroke_color="#eceef1",
+    major_guide_stroke_color="#dde1e6",
+)
+
+# Chart — increased right margin to prevent label truncation
+chart = pygal.XY(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="psychrometric-basic · pygal · pyplots.ai",
+    x_title="Dry-Bulb Temperature (°C)",
+    y_title="Humidity Ratio (g/kg)",
+    show_legend=False,
+    dots_size=0,
+    stroke=True,
+    show_x_guides=True,
+    show_y_guides=True,
+    xrange=(-10, 50),
+    range=(0, 30),
+    x_labels=list(range(-10, 55, 5)),
+    y_labels=list(range(0, 32, 2)),
+    x_labels_major_every=2,
+    y_labels_major_every=5,
+    show_minor_x_labels=True,
+    show_minor_y_labels=True,
+    print_values=False,
+    tooltip_border_radius=8,
+    tooltip_fancy_mode=True,
+    explicit_size=True,
+    spacing=25,
+    margin_bottom=60,
+    margin_top=70,
+    margin_left=80,
+    margin_right=180,
+    truncate_legend=-1,
+    js=[],
+)
+
+# RH curves — saturation (100%) thickest
+for rh in rh_levels:
+    rh_pct = int(rh * 100)
+    w = 5.5 if rh == 1.0 else max(2.2, 1.5 + rh * 2)
+    chart.add(f"{rh_pct}% RH", rh_curves[rh], show_dots=False, stroke_style={"width": w, "linecap": "round"})
+
+# Wet-bulb lines — dashed orange
+for tw in wb_temps:
+    chart.add(
+        f"Tw={tw}°C",
+        wb_lines.get(tw, []),
+        show_dots=False,
+        stroke_style={"width": 2.0, "dasharray": "12, 6", "linecap": "round"},
+    )
+
+# Specific volume lines — dotted purple, increased width for visibility
+for sv in sv_values:
+    chart.add(
+        f"v={sv} m\u00b3/kg",
+        sv_lines.get(sv, []),
+        show_dots=False,
+        stroke_style={"width": 2.2, "dasharray": "4, 8", "linecap": "round"},
+    )
+
+# Enthalpy lines — dash-dot teal
+for h in enthalpy_values:
+    chart.add(
+        f"h={h} kJ/kg",
+        enthalpy_lines.get(h, []),
+        show_dots=False,
+        stroke_style={"width": 1.6, "dasharray": "14, 4, 4, 4", "linecap": "round"},
+    )
+
+# Comfort zone — filled green
+chart.add(
+    "Comfort Zone (20\u201326\u00b0C, 30\u201360% RH)",
+    comfort_pts,
+    show_dots=False,
+    fill=True,
+    stroke_style={"width": 2.5, "linecap": "round"},
+)
+
+# HVAC process — bold red with dots at state points
+chart.add(
+    "Cooling & Dehumidification (A\u2192B)",
+    [
+        {"value": hvac_states[0], "label": "State A: 35\u00b0C, 60% RH"},
+        {"value": hvac_states[1], "label": "State B: 24\u00b0C, 50% RH"},
+    ],
+    show_dots=True,
+    dots_size=14,
+    stroke_style={"width": 4.5, "linecap": "round"},
+)
+
+# Render SVG, then add direct labels via SVG text elements
+svg_content = chart.render(is_unicode=True)
+
+# Coordinate mapping for label placement
+plot_left = 80 + 120  # margin_left + y-axis labels space
+plot_right = 4800 - 180  # width - margin_right
+plot_top = 70 + 80  # margin_top + title space
+plot_bottom = 2700 - 60 - 80  # height - margin_bottom - x-axis space
+
+x_min, x_max = -10, 50
+y_min, y_max = 0, 30
+
+
+def to_svg_x(val):
+    return plot_left + (val - x_min) / (x_max - x_min) * (plot_right - plot_left)
+
+
+def to_svg_y(val):
+    return plot_bottom - (val - y_min) / (y_max - y_min) * (plot_bottom - plot_top)
+
+
+labels_svg = []
+
+# RH labels — positioned along curves where they're readable
+rh_label_temps = {1.0: 16, 0.9: 20, 0.8: 24, 0.7: 27, 0.6: 30, 0.5: 33, 0.4: 36, 0.3: 39, 0.2: 42, 0.1: 45}
+for rh in rh_levels:
+    rh_pct = int(rh * 100)
+    t_l = rh_label_temps[rh]
+    w_l = humidity_ratio(t_l, rh)
+    if 0 < w_l < 30:
+        sx, sy = to_svg_x(t_l), to_svg_y(w_l)
+        labels_svg.append(
+            f'<text x="{sx}" y="{sy - 10}" font-size="34" font-family="Trebuchet MS, sans-serif" '
+            f'fill="#0d3b66" font-weight="bold" text-anchor="middle">{rh_pct}%</text>'
+        )
+
+# Wet-bulb labels — placed at 2/5 along line to separate from enthalpy labels
+for tw in wb_temps:
+    pts = wb_lines.get(tw, [])
+    if len(pts) > 4:
+        idx = 2 * len(pts) // 5  # place label at 2/5 of the line from left
+        p = pts[idx]
+        sx, sy = to_svg_x(p[0]), to_svg_y(p[1])
+        # Compute rotation angle from nearby points for alignment
+        p_prev = pts[max(0, idx - 1)]
+        p_next = pts[min(len(pts) - 1, idx + 1)]
+        dx = to_svg_x(p_next[0]) - to_svg_x(p_prev[0])
+        dy = to_svg_y(p_next[1]) - to_svg_y(p_prev[1])
+        angle = math.degrees(math.atan2(dy, dx))
+        labels_svg.append(
+            f'<text x="{sx}" y="{sy - 10}" font-size="30" font-family="Trebuchet MS, sans-serif" '
+            f'fill="{orange_wb}" font-weight="bold" text-anchor="middle" '
+            f'transform="rotate({angle:.1f},{sx},{sy - 10})">Tw {tw}\u00b0C</text>'
+        )
+
+# Specific volume labels — larger and placed at 1/3 position for visibility
+for sv in sv_values:
+    pts = sv_lines.get(sv, [])
+    if len(pts) > 4:
+        idx = len(pts) // 3
+        p = pts[idx]
+        sx, sy = to_svg_x(p[0]), to_svg_y(p[1])
+        p_prev = pts[max(0, idx - 1)]
+        p_next = pts[min(len(pts) - 1, idx + 1)]
+        dx = to_svg_x(p_next[0]) - to_svg_x(p_prev[0])
+        dy = to_svg_y(p_next[1]) - to_svg_y(p_prev[1])
+        angle = math.degrees(math.atan2(dy, dx))
+        labels_svg.append(
+            f'<text x="{sx}" y="{sy - 10}" font-size="32" font-family="Trebuchet MS, sans-serif" '
+            f'fill="{purple_sv}" font-weight="bold" text-anchor="middle" '
+            f'transform="rotate({angle:.1f},{sx},{sy - 10})">{sv} m\u00b3/kg</text>'
+        )
+
+# Enthalpy labels — placed at 2/3 along line (separated from wet-bulb labels at 2/5)
+for h in [10, 30, 50, 70, 90]:
+    pts = enthalpy_lines.get(h, [])
+    if len(pts) > 4:
+        idx = 2 * len(pts) // 3
+        p = pts[idx]
+        sx, sy = to_svg_x(p[0]), to_svg_y(p[1])
+        p_prev = pts[max(0, idx - 1)]
+        p_next = pts[min(len(pts) - 1, idx + 1)]
+        dx = to_svg_x(p_next[0]) - to_svg_x(p_prev[0])
+        dy = to_svg_y(p_next[1]) - to_svg_y(p_prev[1])
+        angle = math.degrees(math.atan2(dy, dx))
+        labels_svg.append(
+            f'<text x="{sx}" y="{sy - 10}" font-size="30" font-family="Trebuchet MS, sans-serif" '
+            f'fill="{teal_enth}" font-weight="bold" text-anchor="middle" '
+            f'transform="rotate({angle:.1f},{sx},{sy - 10})">h={h} kJ/kg</text>'
+        )
+
+# Comfort zone label
+cz_x, cz_y = 23, humidity_ratio(23, 0.45)
+sx, sy = to_svg_x(cz_x), to_svg_y(cz_y)
+labels_svg.append(
+    f'<text x="{sx}" y="{sy}" font-size="38" font-family="Trebuchet MS, sans-serif" '
+    f'fill="#2e7d32" font-weight="bold" text-anchor="middle">Comfort Zone</text>'
+)
+
+# HVAC state point labels
+sx_a, sy_a = to_svg_x(hvac_states[0][0]), to_svg_y(hvac_states[0][1])
+labels_svg.append(
+    f'<text x="{sx_a + 20}" y="{sy_a - 20}" font-size="36" font-family="Trebuchet MS, sans-serif" '
+    f'fill="#c62828" font-weight="bold" text-anchor="start">A (35\u00b0C, 60%RH)</text>'
+)
+sx_b, sy_b = to_svg_x(hvac_states[1][0]), to_svg_y(hvac_states[1][1])
+labels_svg.append(
+    f'<text x="{sx_b - 20}" y="{sy_b - 20}" font-size="36" font-family="Trebuchet MS, sans-serif" '
+    f'fill="#c62828" font-weight="bold" text-anchor="end">B (24\u00b0C, 50%RH)</text>'
+)
+
+# Insert labels before closing </svg>
+label_block = "\n".join(labels_svg)
+svg_labeled = svg_content.replace("</svg>", f"{label_block}\n</svg>")
+
+with open("plot.html", "w") as f:
+    f.write(svg_labeled)
+
+cairosvg.svg2png(bytestring=svg_labeled.encode("utf-8"), write_to="plot.png")