Add logarithmic scale, custom labels, and fixed-point number formatting

- New Linear/Log mapping toggle; log mode uses geometric major-tick
  values with log-positioned minor ticks (validates min/max > 0)
- Optional comma-separated custom labels that override the calculated
  numbers and set the label count
- Format numbers in fixed-point at 4 significant figures (no more
  scientific notation like "1e+04"), snapping log-scale FP drift to ints
- Bump version to 0.75

web/app.py

@@ -1,4 +1,5 @@
 import base64
+import math
 import pathlib
 from flask import Flask, render_template, request, Response
 from html import escape
@@ -7,7 +8,7 @@ from fontTools.pens.svgPathPen import SVGPathPen
 
 app = Flask(__name__)
 
-VERSION = "0.72"
+VERSION = "0.75"
 
 _LOGO_PATH = pathlib.Path(__file__).parent / 'static' / 'logo.png'
 _LOGO_B64 = base64.b64encode(_LOGO_PATH.read_bytes()).decode()
@@ -49,9 +50,16 @@ def _unit_font_size(unit: str) -> float:
 
 
 def _fmt(v: float) -> str:
-    if v == int(v):
-        return str(int(v))
-    return f"{v:.4g}"
+    # Snap near-integers — log-scale geometric values accumulate FP error
+    # (e.g. 9999.9999), which would otherwise miss the integer case.
+    r = round(v)
+    if abs(v - r) < 1e-9 * max(1.0, abs(v)):
+        return str(int(r))
+    # 4 significant figures in fixed-point notation (never scientific).
+    exp = math.floor(math.log10(abs(v)))
+    decimals = max(0, 4 - 1 - exp)
+    s = f"{v:.{decimals}f}"
+    return s.rstrip('0').rstrip('.') if '.' in s else s
 
 
 def _text_to_paths(text, x, y, font_size, anchor='start', fill='#1a1a1a', wrap_transform=None):
@@ -84,12 +92,48 @@ def _text_to_paths(text, x, y, font_size, anchor='start', fill='#1a1a1a', wrap_t
     return inner
 
 
-def generate_svg(unit, min_val, max_val, range_label, big_ticks, small_ticks, label_count, use_paths=True):
-    big_interval = SCALE_LEN / big_ticks
-    big_xs = [ZERO_X + i * big_interval for i in range(big_ticks + 1)]
+def generate_svg(unit, min_val, max_val, range_label, big_ticks, small_ticks, label_count,
+                 use_paths=True, log_scale=False, custom_labels=None):
+    if log_scale:
+        if min_val <= 0 or max_val <= 0:
+            raise ValueError("Logarithmic scale requires min and max values greater than 0.")
+        lmin, lmax = math.log(min_val), math.log(max_val)
+        lspan = lmax - lmin
+        if lspan == 0:
+            raise ValueError("Logarithmic scale requires min and max values to differ.")
 
-    label_count = max(2, min(label_count, big_ticks + 1))
-    label_indices = [round(k * big_ticks / (label_count - 1)) for k in range(label_count)]
+        def value_to_x(v):
+            return ZERO_X + SCALE_LEN * (math.log(v) - lmin) / lspan
+
+        def major_value(i):
+            return min_val * (max_val / min_val) ** (i / big_ticks)
+    else:
+        span = max_val - min_val
+
+        def value_to_x(v):
+            if span == 0:
+                return ZERO_X
+            return ZERO_X + SCALE_LEN * (v - min_val) / span
+
+        def major_value(i):
+            return min_val + (max_val - min_val) * i / big_ticks
+
+    big_interval = SCALE_LEN / big_ticks
+    # Major ticks are always physically evenly spaced; only their values differ.
+    big_xs = [ZERO_X + i * big_interval for i in range(big_ticks + 1)]
+    big_vals = [major_value(i) for i in range(big_ticks + 1)]
+
+    # Custom labels, when supplied, override the calculated numbers and set the count.
+    if custom_labels:
+        label_count = max(1, min(len(custom_labels), big_ticks + 1))
+        custom_labels = custom_labels[:label_count]
+    else:
+        label_count = max(2, min(label_count, big_ticks + 1))
+
+    if label_count == 1:
+        label_indices = [0]
+    else:
+        label_indices = [round(k * big_ticks / (label_count - 1)) for k in range(label_count)]
 
     parts = [
         '<?xml version="1.0" encoding="UTF-8" standalone="no"?>',
@@ -135,22 +179,23 @@ def generate_svg(unit, min_val, max_val, range_label, big_ticks, small_ticks, la
             f' style="stroke:#1a1a1a;stroke-width:0.555;stroke-linecap:butt" />'
         )
 
-    # Minor ticks
+    # Minor ticks — evenly spaced in value, then positioned by the scale mapping.
+    # On a linear scale this is uniform; on a log scale they crowd toward the high end.
     if small_ticks > 0:
         sub = small_ticks + 1
         for i in range(big_ticks):
             for j in range(1, small_ticks + 1):
-                x = big_xs[i] + j * big_interval / sub
+                v = big_vals[i] + j * (big_vals[i + 1] - big_vals[i]) / sub
+                x = value_to_x(v)
                 parts.append(
                     f'    <line x1="{x:.5f}" y1="{BAR_Y}" x2="{x:.5f}" y2="{BAR_Y - SMALL_H:.5f}"'
                     f' style="stroke:#1a1a1a;stroke-width:0.302;stroke-linecap:butt" />'
                 )
 
     # Numeric labels
-    for idx in label_indices:
+    for k, idx in enumerate(label_indices):
         x = big_xs[idx]
-        val = min_val + (max_val - min_val) * idx / big_ticks
-        label = _fmt(val)
+        label = custom_labels[k] if custom_labels else _fmt(big_vals[idx])
         if use_paths:
             parts.append('    ' + _text_to_paths(label, x, 101.49598, 8.89194, anchor='middle'))
         else:
@@ -178,8 +223,15 @@ def generate():
     small_ticks = max(0, min(20, int(request.form.get('small_ticks', 4))))
     label_count = max(2, int(request.form.get('label_count', 6)))
     use_paths   = request.form.get('output_mode', 'paths') == 'paths'
+    log_scale   = request.form.get('scale_type', 'linear') == 'log'
 
-    svg = generate_svg(unit, min_val, max_val, range_label, big_ticks, small_ticks, label_count, use_paths)
+    custom_labels = [s.strip() for s in request.form.get('labels', '').split(',') if s.strip()] or None
+
+    try:
+        svg = generate_svg(unit, min_val, max_val, range_label, big_ticks, small_ticks,
+                           label_count, use_paths, log_scale, custom_labels)
+    except ValueError as err:
+        return Response(str(err), status=400, mimetype='text/plain')
     return Response(svg, mimetype='image/svg+xml')