Prepare raw data for visualization. This module fires after ingestion and before chart selection. It does not analyze or visualize; it makes data ready for those steps.
When to Use
- Raw data has nulls, duplicates, or inconsistent types
- Data needs aggregation, pivoting, or reshaping for the target visual
- The brief specifies quality concerns
- You are unsure whether the data is visualization-ready
When to Skip
- Data is already clean, typed, and shaped for the target visuals
- The dataset is small and you can verify quality by inspection
- The brief says "data is analysis-ready"
State why you are skipping. Silent skips hide data problems.
Step 1: Profile
Before cleaning, understand what you have. Run these checks on every column:
| Check | What to look for |
|---|---|
| Row count | Is it what you expected? Zero rows = ingestion failed |
| Column types | Dates stored as strings? Numbers as text? |
| Null count | Which columns have nulls? What percentage? |
| Unique count | Cardinality: is this a dimension (low) or measure (high)? |
| Min / Max / Mean | For numeric columns: are ranges plausible? |
| Sample values | Eyeball 5 rows. Do they look right? |
Report the profile as a table before proceeding. The user (or orchestrator) needs to see it to decide what cleaning is needed.
Step 2: Clean
Apply fixes in this order. Each fix is a decision; state what you chose and why.
2a. Duplicates
| Signal | Action |
|---|---|
| Exact duplicate rows | Remove. Log count removed. |
| Near-duplicates (same key, different values) | Flag for user decision. Do not silently drop. |
2b. Null handling
| Strategy | When to use |
|---|---|
| Drop row | Null is in a must-have column and row is < 5% of data |
| Fill with default | Column has an obvious default (e.g., 0 for optional count) |
| Fill with median/mode | Numeric column, nulls are < 10%, distribution is stable |
| Leave as-is | The visual can handle nulls (e.g., gap in a line chart) |
Never fill nulls silently. State the strategy and count affected.
2c. Type coercion
| Problem | Fix |
|---|---|
| Date as string | Parse to date. State the format detected. |
| Number as string | Convert. Check for non-numeric values first. |
| Boolean as string | Map "yes/no", "true/false", "1/0" to boolean. |
| Mixed types in one column | Flag for user. This usually means the schema is wrong. |
2d. Outliers
| Context | Action |
|---|---|
| Outlier is a data error (negative revenue, future date) | Remove or fix. Log. |
| Outlier is real but extreme | Keep. Note it for the visual (it may need axis adjustment). |
| Unsure | Flag for user. Do not remove real data silently. |
Step 3: Transform
Reshape the data to match what the target visual needs.
Aggregation
Aggregate when the raw data is too granular for the visual.
| Visual need | Aggregation |
|---|---|
| Monthly trend | Group by month, sum or average the measure |
| Category comparison | Group by category, sum the measure |
| Proportion | Group by category, compute share of total |
| Distribution | Bin the values, count per bin |
Always state the grain change: "Aggregated from daily to monthly by summing revenue."
Pivot / Unpivot
| Need | Direction |
|---|---|
| One column per category (wide format) | Pivot |
| One row per observation (long format) | Unpivot |
Most visualizations prefer long format. Pivot only when the delivery module explicitly needs wide format (e.g., side-by-side bar chart in ASCII).
Computed columns
Add columns only when the brief requires a metric that is not in the raw data:
- Margin: revenue - cost
- Margin %: (revenue - cost) / revenue
- Growth: (current - previous) / previous
- Share: value / total
Name computed columns clearly. Do not overwrite source columns.
Step 4: Quality Gates
Before passing data downstream, assert:
| Gate | Assertion | Fail action |
|---|---|---|
| No nulls in key columns | Dimensions and must-have measures are null-free | Push back: data is not ready |
| Row count is plausible | Within expected range (not 0, not 10x expected) | Push back: ingestion may have failed |
| Types are correct | Every column has the expected type after coercion | Push back: type fix needed |
| Grain is correct | One row per expected unit (e.g., one per month per region) | Push back: dedup or aggregate needed |
If any gate fails, push back to the orchestrator with a specific message: "Quality gate failed: 12 nulls in revenue column. Recommend: drop rows (< 1% of data)."
Output
Pass to the next module:
- The cleaned, transformed dataset (or a reference to it)
- A one-paragraph data profile summary
- A list of any decisions made (nulls filled, outliers flagged, columns added)
Relationship to Mall Plugins
| Mall plugin | This module's role |
|---|---|
data-analysis |
Runs after this module; assumes clean data |
data-quality-monitoring |
Monitors ongoing quality; this module does initial prep |
data-visualization |
Designs the visual; needs correctly shaped data from this module |
Anti-Patterns
| Do not | Do instead |
|---|---|
| Clean silently | Log every change with count and reason |
| Remove outliers without context | Keep real data; flag for user if unsure |
| Aggregate without stating the grain change | "Aggregated from X to Y by Z" |
| Add computed columns the brief did not ask for | Only add what the questions require |
| Skip profiling because "the data looks fine" | Profile always; it costs one table and catches surprises |