feat(highcharts): implement elbow-curve (#2357)

## Implementation: `elbow-curve` - highcharts

Implements the **highcharts** version of `elbow-curve`.

**File:** `plots/elbow-curve/implementations/highcharts.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20528205442)*

---------

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

Author: github-actions[bot]

plots/elbow-curve/implementations/highcharts.py

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+""" pyplots.ai
+elbow-curve: Elbow Curve for K-Means Clustering
+Library: highcharts unknown | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-26
+"""
+
+import tempfile
+import time
+import urllib.request
+from pathlib import Path
+
+import numpy as np
+from highcharts_core.chart import Chart
+from highcharts_core.options import HighchartsOptions
+from highcharts_core.options.series.area import LineSeries
+from highcharts_core.options.series.scatter import ScatterSeries
+from selenium import webdriver
+from selenium.webdriver.chrome.options import Options
+
+
+# Data - simulate elbow curve from K-means clustering
+np.random.seed(42)
+k_values = list(range(1, 13))
+
+# Simulate realistic inertia values (decreasing with diminishing returns)
+# Using exponential decay with noise for realism
+base_inertia = 15000
+inertia = []
+for k in k_values:
+    # Exponential decay with elbow around k=4
+    decay = base_inertia * np.exp(-0.4 * (k - 1))
+    noise = np.random.uniform(-200, 200)
+    inertia.append(max(500, decay + noise))
+
+# Optimal elbow point at k=4
+optimal_k = 4
+optimal_inertia = inertia[optimal_k - 1]
+
+# Create chart
+chart = Chart(container="container")
+chart.options = HighchartsOptions()
+
+# Chart configuration
+chart.options.chart = {
+    "type": "line",
+    "width": 4800,
+    "height": 2700,
+    "backgroundColor": "#ffffff",
+    "marginBottom": 250,
+    "marginLeft": 250,
+    "marginTop": 120,
+    "marginRight": 100,
+}
+
+# Title
+chart.options.title = {
+    "text": "elbow-curve \u00b7 highcharts \u00b7 pyplots.ai",
+    "style": {"fontSize": "48px", "fontWeight": "bold"},
+}
+
+# X-axis configuration
+chart.options.x_axis = {
+    "title": {"text": "Number of Clusters (k)", "style": {"fontSize": "42px"}, "margin": 30},
+    "labels": {"style": {"fontSize": "32px"}, "y": 40},
+    "categories": [str(k) for k in k_values],
+    "gridLineWidth": 1,
+    "gridLineColor": "#e0e0e0",
+    "gridLineDashStyle": "Dash",
+    "tickInterval": 1,
+}
+
+# Y-axis configuration
+chart.options.y_axis = {
+    "title": {"text": "Inertia (Within-cluster Sum of Squares)", "style": {"fontSize": "42px"}, "margin": 40},
+    "labels": {"style": {"fontSize": "32px"}, "x": -15},
+    "gridLineWidth": 1,
+    "gridLineColor": "#e0e0e0",
+    "gridLineDashStyle": "Dash",
+    "min": 0,
+}
+
+# Legend
+chart.options.legend = {
+    "enabled": True,
+    "itemStyle": {"fontSize": "32px"},
+    "align": "right",
+    "verticalAlign": "top",
+    "layout": "vertical",
+    "x": -80,
+    "y": 80,
+}
+
+# Plot options
+chart.options.plot_options = {
+    "line": {"lineWidth": 5, "marker": {"enabled": True, "radius": 14, "symbol": "circle"}},
+    "scatter": {"marker": {"radius": 20, "symbol": "diamond"}},
+}
+
+# Main elbow curve line series
+line_series = LineSeries()
+line_series.name = "Inertia"
+line_series.data = [[i, round(v, 1)] for i, v in enumerate(inertia)]
+line_series.color = "#306998"
+line_series.marker = {"fillColor": "#306998", "lineWidth": 3, "lineColor": "#ffffff"}
+chart.add_series(line_series)
+
+# Optimal point marker
+optimal_series = ScatterSeries()
+optimal_series.name = f"Optimal k = {optimal_k}"
+optimal_series.data = [[optimal_k - 1, round(optimal_inertia, 1)]]
+optimal_series.color = "#FFD43B"
+optimal_series.marker = {
+    "radius": 24,
+    "symbol": "diamond",
+    "fillColor": "#FFD43B",
+    "lineWidth": 4,
+    "lineColor": "#306998",
+}
+chart.add_series(optimal_series)
+
+# Add annotation for elbow point
+chart.options.annotations = [
+    {
+        "labels": [
+            {
+                "point": {"x": optimal_k - 1, "y": optimal_inertia, "xAxis": 0, "yAxis": 0},
+                "text": f"Elbow Point (k={optimal_k})",
+                "style": {"fontSize": "32px", "fontWeight": "bold"},
+                "backgroundColor": "rgba(255, 212, 59, 0.9)",
+                "borderColor": "#306998",
+                "borderWidth": 3,
+                "borderRadius": 8,
+                "padding": 15,
+                "y": -60,
+            }
+        ]
+    }
+]
+
+# Tooltip
+chart.options.tooltip = {
+    "style": {"fontSize": "24px"},
+    "headerFormat": "<b>k = {point.key}</b><br/>",
+    "pointFormat": "Inertia: {point.y:.1f}",
+}
+
+# Download Highcharts JS (required for headless Chrome)
+highcharts_url = "https://code.highcharts.com/highcharts.js"
+with urllib.request.urlopen(highcharts_url, timeout=30) as response:
+    highcharts_js = response.read().decode("utf-8")
+
+# Download annotations module for elbow point label
+annotations_url = "https://code.highcharts.com/modules/annotations.js"
+with urllib.request.urlopen(annotations_url, timeout=30) as response:
+    annotations_js = response.read().decode("utf-8")
+
+# Generate HTML with inline scripts
+html_str = chart.to_js_literal()
+html_content = f"""<!DOCTYPE html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <script>{highcharts_js}</script>
+    <script>{annotations_js}</script>
+</head>
+<body style="margin:0;">
+    <div id="container" style="width: 4800px; height: 2700px;"></div>
+    <script>{html_str}</script>
+</body>
+</html>"""
+
+# Write temp HTML and take screenshot
+with tempfile.NamedTemporaryFile(mode="w", suffix=".html", delete=False, encoding="utf-8") as f:
+    f.write(html_content)
+    temp_path = f.name
+
+chrome_options = Options()
+chrome_options.add_argument("--headless")
+chrome_options.add_argument("--no-sandbox")
+chrome_options.add_argument("--disable-dev-shm-usage")
+chrome_options.add_argument("--disable-gpu")
+chrome_options.add_argument("--window-size=4800,2800")
+
+driver = webdriver.Chrome(options=chrome_options)
+driver.get(f"file://{temp_path}")
+time.sleep(5)
+driver.save_screenshot("plot.png")
+driver.quit()
+
+# Also save HTML for interactive version
+with open("plot.html", "w", encoding="utf-8") as f:
+    f.write(html_content)
+
+Path(temp_path).unlink()

plots/elbow-curve/metadata/highcharts.yaml

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+library: highcharts
+specification_id: elbow-curve
+created: '2025-12-26T19:37:11Z'
+updated: '2025-12-26T19:43:59Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20528205442
+issue: 0
+python_version: 3.13.11
+library_version: unknown
+preview_url: https://storage.googleapis.com/pyplots-images/plots/elbow-curve/highcharts/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/elbow-curve/highcharts/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/elbow-curve/highcharts/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent visual clarity with appropriate font sizes and marker visibility
+  - Proper implementation of elbow curve with clear elbow point annotation using Highcharts
+    annotations module
+  - Colorblind-safe color scheme with good contrast
+  - Well-structured code following KISS principles with reproducible random seed
+  - Correctly uses both LineSeries and ScatterSeries for different purposes
+  - Title follows the required format exactly
+  weaknesses:
+  - Legend positioned in upper right corner could be better integrated with the chart
+    area
+  - Could leverage more Highcharts-specific interactive features for the interactive
+    HTML output