Chart-Question Fit.
Match the chart to the analytical question, not the data shape.
'How much?' → bar. 'When did it change?' → line. 'What share?' → treemap. 'How does it flow?' → Sankey. 'Who's winning?' → bump chart. The chart type should be chosen by the analytical question, not the data shape.
Heer & Bostock, 2010 · Crowdsourcing Graphical Perception
Charts that apply this principle
Nightingale Chart
Polar area chart with wedges of equal angle but varying radius, a polar histogram.
Polar Area Chart
Smooth continuous area plotted in polar coordinates. Ideal for cyclic data (24-hour cycles, compass directions, monthly seasonality).
Combo Chart
Overlays bars and lines with dual Y-axes to show volume and rate together.
Dendrogram
Tree diagram showing hierarchical clustering - branches merge at similarity thresholds.
Gauge Chart
Semicircular meter showing a single value against a scale, like a car speedometer.
Radar / Spider Chart
Variables plotted on radial axes from a center point, showing multivariate profiles.
Candlestick / OHLC
Shows open, high, low, close for financial data. Each candle is one time period.
Event Timeline
Discrete events as point markers on a single time axis, optionally grouped into lanes — distinct from a Gantt (no bar lengths / durations) and from sparkline-with-annotations (events are the primary mark, not the overlay).
Fan Chart
Line chart with expanding confidence bands that shows forecast uncertainty.
Index Chart
Line chart rebased so every series starts at 100. Reads as 'normalized growth from baseline,' making relative change comparable across series of very different absolute magnitudes.
Line Chart
Connects data points with lines to show trends over time. The workhorse of time-series visualization.
Step Chart
Line chart with horizontal steps between points. No interpolation between values.