HeatmapBuilder

A Ruby gem that generates embeddable SVG heatmap visualizations with GitHub-style calendar layouts and linear progress indicators. Perfect for Rails applications and any project that needs to display activity data in a visual format.

• GitHub-style calendar layouts for date-based data.
• Linear heatmaps.
• Vector-based output (SVG) for crisp rendering at any resolution.
• Optional numeric values displayed in each cell.
• Use pre-calculated scores or raw numeric values - automatic mapping to color scales.
• Custom value-to-score conversion functions for advanced scoring logic.
• Parametric everything: customize cell size, spacing, colors, fonts, etc.
• Rounded corners (and circular cells, if you're into that kind of thing).
• Dynamic palette generation from two colors or manually-specified colors.
• OKLCH color interpolation for clean color transitions and perceptual uniformity.
• Zero dependencies.

Add this line to your application's Gemfile:

gem 'heatmap-builder'

And then execute:

$ bundle

Or install it yourself as:

$ gem install heatmap-builder

require 'heatmap-builder'

# Generate SVG for daily scores
scores = [0, 1, 2, 3, 4, 5, 2, 1]
svg = HeatmapBuilder.build_linear(scores: scores)

# In a Rails view
<%= raw HeatmapBuilder.build_linear(scores: @daily_scores) %>

# GitHub-style calendar heatmap
scores_by_date = {
  '2024-01-01' => 2,
  '2024-01-02' => 4,
  '2024-01-03' => 1,
  # ... more dates
}

svg = HeatmapBuilder.build_calendar(scores: scores_by_date)

You must provide either scores: or values: (but not both). All other options are optional keyword arguments with sensible defaults.

Data options:

• scores - Array of pre-calculated scores (integers from 0 to number of colors minus 1). Required if values is not provided.
• values - Array of arbitrary numeric values to be automatically mapped to scores. Required if scores is not provided. See Using Raw Values Instead of Scores.

Value-to-score conversion options (only used with values:):

• value_min - Minimum boundary for value-to-score mapping. Defaults to the minimum value in your data.
• value_max - Maximum boundary for value-to-score mapping. Defaults to the maximum value in your data.
• value_to_score - Custom callable for value-to-score conversion. Receives value:, index:, min:, max:, max_score: parameters and must return an integer between 0 and max_score. See Custom Scoring Logic for details.

Appearance options:

• cell_size - Size of each square in pixels. Defaults to 10.
• cell_spacing - Space between squares in pixels. Defaults to 1.
• font_size - Font size for score text in pixels. Defaults to 8.
• border_width - Border width around each cell in pixels. Defaults to 1.
• corner_radius - Corner radius for rounded cells. Must be between 0 (square corners) and floor(cell_size/2) (circular cells). Values outside this range are automatically clamped. Defaults to 0.
• text_color - Color of score text as a hex string. Defaults to "#000000" (black).

Color options:

• colors - Color palette for the heatmap. Can be a predefined palette constant (e.g., HeatmapBuilder::GITHUB_GREEN), an array of hex color strings (e.g., %w[#ebedf0 #9be9a8 #40c463]), or a hash for OKLCH interpolation (e.g., { from: "#ebedf0", to: "#216e39", steps: 5 }). Defaults to HeatmapBuilder::GITHUB_GREEN. See Predefined Color Palettes and Dynamic Palettes Generation.

You must provide either scores: or values: (but not both). All other options are optional keyword arguments with sensible defaults.

Data options:

• scores - Hash of pre-calculated scores by date (integers from 0 to number of colors minus 1). Keys can be Date objects or date strings (e.g., '2024-01-01'). Required if values is not provided.
• values - Hash of arbitrary numeric values by date to be automatically mapped to scores. Keys can be Date objects or date strings. Required if scores is not provided. See Using Raw Values Instead of Scores.

Value-to-score conversion options (only used with values:):

• value_min - Minimum boundary for value-to-score mapping. Defaults to the minimum value in your data.
• value_max - Maximum boundary for value-to-score mapping. Defaults to the maximum value in your data.
• value_to_score - Custom callable for value-to-score conversion. Receives value:, date:, min:, max:, max_score: parameters and must return an integer between 0 and max_score. See Custom Scoring Logic for details.

Appearance options:

• cell_size - Size of each square in pixels. Defaults to 12.
• cell_spacing - Space between squares in pixels. Defaults to 1.
• font_size - Font size for labels in pixels. Defaults to 8.
• border_width - Border width around each cell in pixels. Defaults to 1.
• corner_radius - Corner radius for rounded cells. Must be between 0 (square corners) and floor(cell_size/2) (circular cells). Values outside this range are automatically clamped. Defaults to 0.
• text_color - Color of label text as a hex string. Defaults to "#000000" (black).

Color options:

• colors - Color palette for the heatmap. Can be a predefined palette constant (e.g., HeatmapBuilder::GITHUB_GREEN), an array of hex color strings (e.g., %w[#ebedf0 #9be9a8 #40c463]), or a hash for OKLCH interpolation (e.g., { from: "#ebedf0", to: "#216e39", steps: 5 }). Defaults to HeatmapBuilder::GITHUB_GREEN. See Predefined Color Palettes and Dynamic Palettes Generation.

Calendar-specific options:

• start_of_week - First day of the week. One of :sunday, :monday, :tuesday, :wednesday, :thursday, :friday, :saturday. Defaults to :monday.
• month_spacing - Extra horizontal space between months in pixels. Defaults to 5.
• show_month_labels - Show month names at the top of the calendar. Defaults to true.
• show_day_labels - Show day abbreviations on the left side of the calendar. Defaults to true.
• show_outside_cells - Show cells outside the date range with inactive styling. Defaults to false.

Internationalization options:

• day_labels - Array of day abbreviations starting from Sunday (7 elements). Defaults to %w[S M T W T F S]. See I18n.
• month_labels - Array of month abbreviations from January to December (12 elements). Defaults to %w[Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec]. See I18n.

A score is an integer (0 to N-1) that maps directly to a color in your palette. For example, with 5 colors, valid scores are 0-4.

Instead of pre-calculating scores, you can provide raw numeric values (like 45.2, 78, 1000) and let the builder automatically map them to scores using linear distribution:

# Linear heatmap with automatic score calculation
values = [10, 25, 50, 75, 100]
svg = HeatmapBuilder.build_linear(
  values: values,
  value_min: 0,    # Optional: explicitly set minimum (defaults to actual min)
  value_max: 100   # Optional: explicitly set maximum (defaults to actual max)
)

# Calendar heatmap with automatic score calculation
values_by_date = {
  Date.new(2024, 1, 1) => 45.2,
  Date.new(2024, 1, 2) => 78.5,
  Date.new(2024, 1, 3) => 12.0
}

svg = HeatmapBuilder.build_calendar(
  values: values_by_date,
  value_min: 0,
  value_max: 100
)

The builder will automatically:

• Calculate min/max boundaries from your data if not specified
• Map values to color scores using linear distribution
• Clamp values outside the boundaries
• Handle nil values by treating them as the minimum boundary

By default, values are mapped to scores using linear distribution. You can provide a custom value-to-score conversion function for different behaviors like logarithmic scales, exponential curves, or custom thresholds.

The callable receives these parameters:

• value: - The current value being converted
• index: or date: - The position in the data (linear heatmaps use index:, calendar heatmaps use date:)
• min: - The minimum boundary value
• max: - The maximum boundary value
• max_score: - The maximum valid score (color palette length minus 1)

The function must return an integer between 0 and max_score.

Custom scoring logic - linear distribution example:

linear_formula = ->(value:, index:, min:, max:, max_score:) {
  ((value - min) / (max - min) * max_score).round
}

svg = HeatmapBuilder.build_linear(
  values: [10, 20, 30],
  value_to_score: linear_formula
)

Logarithmic scale for data with wide range (e.g., 1 to 10000):

logarithmic_formula = ->(value:, index:, min:, max:, max_score:) {
  return 0 if value <= 0 || min <= 0

  log_value = Math.log10(value)
  log_min = Math.log10(min)
  log_max = Math.log10(max)

  ((log_value - log_min) / (log_max - log_min) * max_score).round.clamp(0, max_score)
}

svg = HeatmapBuilder.build_linear(
  values: [1, 10, 100, 1000, 10000],
  value_to_score: logarithmic_formula
)

HeatmapBuilder.build_linear(scores: scores, colors: HeatmapBuilder::GITHUB_GREEN)

HeatmapBuilder.build_calendar(scores: calendar_data, colors: HeatmapBuilder::GITHUB_GREEN)

HeatmapBuilder.build_linear(scores: scores, colors: HeatmapBuilder::BLUE_OCEAN)

HeatmapBuilder.build_calendar(scores: calendar_data, colors: HeatmapBuilder::BLUE_OCEAN)

HeatmapBuilder.build_linear(scores: scores, colors: HeatmapBuilder::WARM_SUNSET)

HeatmapBuilder.build_calendar(scores: calendar_data, colors: HeatmapBuilder::WARM_SUNSET)

HeatmapBuilder.build_linear(scores: scores, colors: HeatmapBuilder::PURPLE_VIBES)

HeatmapBuilder.build_calendar(scores: calendar_data, colors: HeatmapBuilder::PURPLE_VIBES)

HeatmapBuilder.build_linear(scores: scores, colors: HeatmapBuilder::RED_TO_GREEN)

HeatmapBuilder.build_calendar(scores: calendar_data, colors: HeatmapBuilder::RED_TO_GREEN)

Generate custom color palettes from any two colors using OKLCH color space for superior color interpolation:

# Generate a 5-step palette from electric cyan to hot magenta
neon_gradient = {
  from: "#00FFFF",
  to: "#FF1493",
  steps: 5
}

svg = HeatmapBuilder.build_linear(scores: scores, colors: neon_gradient)

The OKLCH color space ensures perceptually uniform color transitions, making gradients appear smooth and natural to the human eye.

Both linear and calendar heatmaps support rounded corners using the corner_radius option.

A typical value is around 2 pixels for a subtle rounded effect:

# Linear heatmap with rounded corners
HeatmapBuilder.build_linear(
  scores: scores,
  corner_radius: 2,
  cell_size: 18
)

The corner_radius value must be between 0 (square corners) and floor(cell_size/2). Values outside this range are automatically clamped to the valid range (negative values become 0, values exceeding the maximum become floor(cell_size/2)). Maximum radius values render circular cells:

# Linear heatmap with max radius rounded corners - circular cells
HeatmapBuilder.build_linear(
  scores: scores,
  corner_radius: 9,
  cell_size: 18
)

Calendar heatmap examples:

# Calendar heatmap with rounded corners
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  corner_radius: 2,
  cell_size: 14
)

# Calendar heatmap with max radius rounded corners - circular cells
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  corner_radius: 7,
  cell_size: 14
)

Calendar heatmaps support internationalization by customizing the day_labels and month_labels options:

# French calendar
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  day_labels: %w[D L M M J V S],  # Dimanche, Lundi, Mardi, etc.
  month_labels: %w[Jan Fév Mar Avr Mai Jun Jul Aoû Sep Oct Nov Déc]
)

# German calendar
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  day_labels: %w[S M D M D F S],  # Sonntag, Montag, Dienstag, etc.
  month_labels: %w[Jan Feb Mär Apr Mai Jun Jul Aug Sep Okt Nov Dez]
)

# Italian calendar
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  day_labels: %w[D L M M G V S],  # Domenica, Lunedì, Martedì, etc.
  month_labels: %w[Gen Feb Mar Apr Mag Giu Lug Ago Set Ott Nov Dic]
)

# Spanish calendar
HeatmapBuilder.build_calendar(
  scores: calendar_data,
  day_labels: %w[D L M X J V S],  # Domingo, Lunes, Martes, etc.
  month_labels: %w[Ene Feb Mar Abr May Jun Jul Ago Sep Oct Nov Dic]
)

The day_labels array should contain 7 elements starting from Sunday, and month_labels should contain 12 elements for January through December.

After checking out the repo, run bin/setup to install development dependencies.

# Run all tests
rake test

# Run tests with code linting
rake

# Update test snapshots after making intentional changes to output
rake update_snapshots

To generate all example SVG files you see in this readme:

bin/generate_examples

Bug reports and pull requests are welcome on GitHub at https://github.com/dreikanter/heatmap-builder. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

The gem is available as open source under the terms of the MIT License.

Everyone interacting in the HeatmapBuilder project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.