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chart-selection.md

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+# Chart Selection
+
+Choose visuals based on the question, not on what is easiest to render.
+
+## Question to Chart Map
+
+| Question | Preferred chart | Notes |
+|----------|-----------------|-------|
+| How is a metric changing over time? | line chart | annotate structural breaks and missing data |
+| Which groups are highest or lowest? | sorted bar chart | keep a shared baseline |
+| How is the distribution shaped? | histogram or box plot | avoid average-only summaries |
+| Are two variables related? | scatter plot | show trend and outliers separately |
+| How do parts contribute to the whole? | stacked bar with totals | keep category count low |
+| Where are users dropping? | funnel chart | define the time window explicitly |
+| How do cohorts retain over time? | cohort table or heatmap | show cohort size alongside retention |
+
+## Default Rules
+
+- Bars start at zero unless there is a strong reason not to.
+- Show underlying counts next to percentages when denominators are small.
+- Prefer direct labels over legends when possible.
+- Use one chart per decision question, not one chart per available metric.
+
+## Visual Anti-Patterns
+
+- Pie charts with many slices -> comparisons become guesswork.
+- Dual-axis charts -> viewers infer relationships that are not there.
+- Cumulative-only charts -> hide recent deterioration or recovery.
+- Truncated bar axes -> exaggerate small differences.
+- Stacked areas with many categories -> impossible to compare layers.
+
+## Before Shipping a Chart
+
+Check:
+
+1. What decision question this chart answers.
+2. Whether the baseline is visible.
+3. Whether the grain and time window match the narrative.
+4. Whether annotations explain outages, launches, or missing data.
+5. Whether a table would be clearer than the chart.