---

name: Implement Double Number Line Question description: Create D3 questions with double number lines showing proportional relationships. Students complete missing values on parallel number lines.

Implement Double Number Line Question

Use this skill when creating questions where students:

• Complete missing values on double number lines
• Work with proportional relationships shown on parallel number lines
• Fill in blanks on two aligned number lines showing equivalent ratios

When to Use This Pattern

Perfect for:

• "Complete the double number line showing days and pies"
• "Fill in the missing values on the number lines"
• Questions with two parallel number lines showing proportional relationships
• Equivalent ratio visualization with number lines

Not suitable for:

• Single number lines → use custom implementation
• Graph coordinate planes → use implement-dynamic-graph-question
• Simple tables → use implement-table-question

Components Required

Copy these from .claude/skills/question-types/:

Required

• implement-double-number-line-question/snippets/double-number-line.js → Complete double number line implementation
• snippets/cards/standard-card.js → createStandardCard()

Optional

• snippets/cards/explanation-card.js → createExplanationCard() - For student explanations

Quick Start

1. Review the pattern guide: PATTERN.md
2. Study the snippet: snippets/double-number-line.js
3. Copy from the working example:

   cat courses/IM-8th-Grade/modules/Unit-3/assignments/117-Equivalent-Ratios/questions/02/attachments/chart.js
   

Implementation Steps

1. Define State

function createDefaultState() {
  return {
    dnl1: "", // First blank on top number line
    dnl2: "", // Second blank on top number line
    dnl3: "", // First blank on bottom number line
    dnl4: "", // Second blank on bottom number line
    finalAnswer: "" // Optional: answer based on the number lines
  };
}

2. Copy the Double Number Line Snippet

Inline the complete double-number-line.js snippet into your chart.js file.

3. Configure and Call

function buildLayout(d3, containerSelector) {
  const container = d3.select(containerSelector);
  container.html("");

  // Create double number line
  const dnlInputs = createDoubleNumberLine(container, {
    svgWidth: 700,
    svgHeight: 200,
    topLabel: "Days",
    bottomLabel: "Pies",
    topValues: ["0", "3", "6", "9", null, null],
    bottomValues: ["0", "8", "16", "24", null, null],
    inputPositions: {
      top: [{ index: 4, key: "dnl1" }, { index: 5, key: "dnl2" }],
      bottom: [{ index: 4, key: "dnl3" }, { index: 5, key: "dnl4" }]
    }
  });

  // Store for interactivity control
  window.dnlInputs = dnlInputs;
}

4. Implement Interactivity Control

function setInteractivity(enabled) {
  if (window.dnlInputs) {
    window.dnlInputs.setInteractivity(enabled);
  }
}

Configuration Options

Basic Configuration

{
  svgWidth: 700,        // SVG width (default: 700)
  svgHeight: 200,       // SVG height (default: 200)
  lineY1: 60,          // Top line Y position (default: 60)
  lineY2: 140,         // Bottom line Y position (default: 140)
  lineStartX: 50,      // Line start X (default: 50)
  lineEndX: 650,       // Line end X (default: 650)
  tickLength: 10,      // Tick mark length (default: 10)
  topLabel: "Days",    // Top line label
  bottomLabel: "Pies", // Bottom line label
  numPositions: 6      // Number of tick marks (default: 6)
}

Value Configuration

{
  topValues: ["0", "3", "6", "9", null, null],    // null = input box
  bottomValues: ["0", "8", "16", "24", null, null],
  inputPositions: {
    top: [
      { index: 4, key: "dnl1" },  // Position 4, state key "dnl1"
      { index: 5, key: "dnl2" }   // Position 5, state key "dnl2"
    ],
    bottom: [
      { index: 4, key: "dnl3" },
      { index: 5, key: "dnl4" }
    ]
  }
}

Critical Implementation Pattern: foreignObject

The snippet uses SVG foreignObject for input positioning. This is the ONLY reliable method.

Why foreignObject?

✅ Correct: Uses same coordinate system as SVG text labels ✅ Correct: Direct SVG coordinates with centering formula ✅ Correct: Perfect alignment with tick marks

❌ Wrong: Div overlay with percentage positioning ❌ Wrong: Mixing coordinate systems

Centering Formula

For an input of width W and height H, to center it on SVG coordinate (x, y):

.attr("x", x - W/2)  // Subtract half the width
.attr("y", y - H/2)  // Subtract half the height

Example:

• Input is 60px wide, 32px tall
• Want to center on (530, 35)
• Set x = 530 - 30 = 500
• Set y = 35 - 16 = 19

Common Customizations

Different Number of Ticks

{
  numPositions: 8,  // 8 ticks instead of 6
  topValues: ["0", "2", "4", "6", null, null, "14", "16"],
  bottomValues: ["0", "5", "10", "15", null, null, "35", "40"]
}

Different Input Positions

{
  topValues: ["0", "3", null, null, "12", null],
  inputPositions: {
    top: [
      { index: 2, key: "dnl1" },
      { index: 3, key: "dnl2" },
      { index: 5, key: "dnl3" }
    ],
    bottom: [
      { index: 2, key: "dnl4" },
      { index: 3, key: "dnl5" },
      { index: 5, key: "dnl6" }
    ]
  }
}

With Optional Table

Many double number line questions include a table showing the relationship first:

// Before creating the double number line
const tableCard = createStandardCard(container, {
  title: "Ratio Table",
  content: tableHtml
});

// Then create the double number line
const dnlInputs = createDoubleNumberLine(container, config);

Implementation Checklist

• State includes keys for all input boxes (dnl1, dnl2, dnl3, dnl4)
• Copied double-number-line.js snippet into chart.js
• Configured topLabel and bottomLabel
• Configured topValues and bottomValues (null for input positions)
• Configured inputPositions with correct indices and state keys
• Called createDoubleNumberLine() in buildLayout()
• Stored return value for interactivity control
• Implemented setInteractivity() function
• sendChartState() updates parent on input changes
• Tested in browser to verify alignment

DO NOT

❌ Use div overlay with percentage positioning ❌ Mix coordinate systems (SVG and CSS percentages) ❌ Hardcode pixel positions without centering formula ❌ Use transform: translate() for positioning inputs ❌ Forget to subtract half width/height for centering

DO

✅ Always use SVG foreignObject for input positioning ✅ Use same coordinate system as SVG text labels ✅ Apply centering formula: x - width/2, y - height/2 ✅ Position inputs at tick mark X-coordinates ✅ Test alignment in actual browser ✅ Document Y-coordinate choices for maintainability

Related Patterns

• implement-table-question - For tabular data entry
• implement-static-graph-question - For coordinate graphs with tables
• implement-custom-d3-question - For other custom SVG interactions

Complete Working Example

courses/IM-8th-Grade/modules/Unit-3/assignments/117-Equivalent-Ratios/questions/02/attachments/chart.js

This example includes:

• Static table showing the relationship
• Double number line with 4 input boxes
• Final answer input based on the completed number lines
• Two-part structure (Part 1: Complete number line, Part 2: Answer question)