feat(highcharts): implement indicator-ema (#3690)

## Implementation: `indicator-ema` - highcharts

Implements the **highcharts** version of `indicator-ema`.

**File:** `plots/indicator-ema/implementations/highcharts.py`

**Parent Issue:** #3652

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20886979938)*

---------

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/indicator-ema/implementations/highcharts.py

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+""" pyplots.ai
+indicator-ema: Exponential Moving Average (EMA) Indicator Chart
+Library: highcharts unknown | Python 3.13.11
+Quality: 91/100 | Created: 2026-01-11
+"""
+
+import tempfile
+import time
+import urllib.request
+from pathlib import Path
+
+import numpy as np
+import pandas as pd
+from highcharts_core.chart import Chart
+from highcharts_core.options import HighchartsOptions
+from highcharts_core.options.series.area import LineSeries
+from selenium import webdriver
+from selenium.webdriver.chrome.options import Options
+
+
+# Generate realistic stock price data
+np.random.seed(42)
+n_days = 120
+
+# Start with a base price and generate realistic daily returns
+base_price = 150.0
+daily_returns = np.random.normal(0.001, 0.02, n_days)  # Small positive drift with volatility
+
+# Generate price series with cumulative returns
+prices = [base_price]
+for ret in daily_returns[1:]:
+    prices.append(prices[-1] * (1 + ret))
+prices = np.array(prices)
+
+# Create date range (trading days)
+dates = pd.date_range(start="2024-06-01", periods=n_days, freq="B")
+
+
+# Calculate EMAs
+def calc_ema(prices, span):
+    """Calculate exponential moving average."""
+    ema = np.zeros(len(prices))
+    multiplier = 2 / (span + 1)
+    ema[0] = prices[0]
+    for i in range(1, len(prices)):
+        ema[i] = prices[i] * multiplier + ema[i - 1] * (1 - multiplier)
+    return ema
+
+
+ema_12 = calc_ema(prices, 12)
+ema_26 = calc_ema(prices, 26)
+
+# Convert dates to timestamps for Highcharts
+timestamps = [int(d.timestamp() * 1000) for d in dates]
+
+# Create chart
+chart = Chart(container="container")
+chart.options = HighchartsOptions()
+
+# Chart configuration
+chart.options.chart = {
+    "type": "line",
+    "width": 4800,
+    "height": 2700,
+    "backgroundColor": "#ffffff",
+    "spacingTop": 60,
+    "spacingBottom": 100,
+    "spacingLeft": 80,
+    "spacingRight": 80,
+}
+
+# Title
+chart.options.title = {
+    "text": "indicator-ema · highcharts · pyplots.ai",
+    "style": {"fontSize": "56px", "fontWeight": "bold"},
+    "margin": 40,
+}
+
+# Subtitle
+chart.options.subtitle = {
+    "text": "Stock Price with 12-day and 26-day EMAs",
+    "style": {"fontSize": "36px", "color": "#666666"},
+}
+
+# X-axis configuration (datetime)
+chart.options.x_axis = {
+    "type": "datetime",
+    "title": {"text": "Date", "style": {"fontSize": "36px"}, "margin": 25},
+    "labels": {"style": {"fontSize": "28px"}},
+    "gridLineWidth": 1,
+    "gridLineColor": "rgba(0, 0, 0, 0.1)",
+    "tickInterval": 30 * 24 * 3600 * 1000,  # Monthly ticks
+    "dateTimeLabelFormats": {"month": "%b %Y"},
+}
+
+# Y-axis configuration
+chart.options.y_axis = {
+    "title": {"text": "Price (USD)", "style": {"fontSize": "36px"}, "margin": 25},
+    "labels": {"style": {"fontSize": "28px"}, "format": "${value:.0f}"},
+    "gridLineWidth": 1,
+    "gridLineColor": "rgba(0, 0, 0, 0.1)",
+}
+
+# Legend
+chart.options.legend = {"enabled": True, "itemStyle": {"fontSize": "32px"}, "margin": 30}
+
+# Tooltip
+chart.options.tooltip = {
+    "shared": True,
+    "crosshairs": True,
+    "style": {"fontSize": "28px"},
+    "valueDecimals": 2,
+    "valuePrefix": "$",
+    "xDateFormat": "%b %d, %Y",
+}
+
+# Plot options
+chart.options.plot_options = {
+    "line": {"lineWidth": 5, "marker": {"enabled": False}, "states": {"hover": {"lineWidth": 6}}}
+}
+
+# Price series (prominent)
+price_series = LineSeries()
+price_series.name = "Close Price"
+price_series.data = [[t, round(p, 2)] for t, p in zip(timestamps, prices, strict=True)]
+price_series.color = "#306998"  # Python Blue
+price_series.line_width = 5
+price_series.z_index = 3
+chart.add_series(price_series)
+
+# EMA 12 series
+ema12_series = LineSeries()
+ema12_series.name = "EMA 12"
+ema12_series.data = [[t, round(e, 2)] for t, e in zip(timestamps, ema_12, strict=True)]
+ema12_series.color = "#FFD43B"  # Python Yellow
+ema12_series.line_width = 3
+ema12_series.dash_style = "Solid"
+ema12_series.z_index = 2
+chart.add_series(ema12_series)
+
+# EMA 26 series
+ema26_series = LineSeries()
+ema26_series.name = "EMA 26"
+ema26_series.data = [[t, round(e, 2)] for t, e in zip(timestamps, ema_26, strict=True)]
+ema26_series.color = "#9467BD"  # Purple
+ema26_series.line_width = 3
+ema26_series.dash_style = "ShortDash"
+ema26_series.z_index = 1
+chart.add_series(ema26_series)
+
+# Download Highcharts JS for inline embedding
+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")
+
+# 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>
+</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=5000,3000")
+
+driver = webdriver.Chrome(options=chrome_options)
+driver.get(f"file://{temp_path}")
+time.sleep(5)
+
+# Screenshot the chart container element for exact dimensions
+container = driver.find_element("id", "container")
+container.screenshot("plot.png")
+driver.quit()
+
+Path(temp_path).unlink()
+
+# Save HTML for interactive version (using CDN for portability)
+interactive_html = f"""<!DOCTYPE html>
+<html>
+<head>
+    <meta charset="utf-8">
+    <title>indicator-ema · highcharts · pyplots.ai</title>
+    <script src="https://code.highcharts.com/highcharts.js"></script>
+</head>
+<body style="margin:0;">
+    <div id="container" style="width: 100%; height: 100vh;"></div>
+    <script>{html_str}</script>
+</body>
+</html>"""
+with open("plot.html", "w", encoding="utf-8") as f:
+    f.write(interactive_html)