Histogram and Multi-Series Histogram

[stonematt]

📊 Add Updated Histogram Charts (Single & Multi-Series)

This PR includes one replacement and one new chart:

---

✅ histogram_chart/ (v2)

• Replaces the original histogram chart with a backward-compatible version
• Supports:
  • Configurable number of bins
  • Optional chart title override
  • Dynamic axis labels based on selected fields
• Retains the single-series bar chart style (no stacking/grouping toggle)

---

➕ histogram_multi_series/

• New step-area chart for comparing histogram distributions across categories
• Supports:
  • Grouped (overlay) or stacked layout via select toggle
  • Dynamic axis and legend labels
  • Custom chart title and bin count
• Great for comparing time-based or categorical shifts
• Includes embedded screenshot in README

---

Each folder includes:

• README.md with field/option documentation
• vgl.aml Vega-Lite spec
• Optional preview image for visual reference

Let me know if you'd like these added to the example gallery or extended with percent-normalized mode in a follow-up.