Chart Type for Heatmap / Intensity. Keywords: heatmap, heat-map, intensity, density, matrix, calendar. Best Chart: Heat Map or Choropleth. Secondary Options: Grid Heat Map, Bubble Heat. When to Use: Showing intensity/density across a 2D grid; time-based patterns (e.g., activity by hour × day). When NOT to Use: Fewer than 20 cells (use bar); user needs to read exact values; colorblind users without pattern fallback. Data Volume Threshold: Up to 10,000 cells efficiently; beyond that aggregate; calendar heatmap: 365 cells max per SVG. Color Guidance: Gradient: Cool (blue) to Hot (red). Divergent scale for ±data. Always include numeric color legend. Accessibility: B — Pattern overlay for colorblind users. Numerical value on hover. Legend must include scale ticks.. A11y Fallback: Numerical overlay on hover; downloadable grid table with row/column labels. Library: D3.js, Plotly, ApexCharts. Interactive Level: Hover + Zoom.