Design principles
The rules VisDelta is built on, and which a chart module must keep.
These principles explain behaviour you will observe in every chart type. The complete normative text, including vocabulary governance and transition contracts, is the language framework in the repository.
Every frame is true
Marks always agree with the scales, ticks, grid lines and titles that explain them, at every progress value. Exit marks before changing the scale; change the scale before entering marks.
State is fact; history is not a script
Planning consumes endpoint states, not the sequence of builder calls that produced them. Two chains that reach the same normalized state produce the same transition. Transform order is still part of a state's meaning: sorting before a limit can change which rows remain.
Data meaning is explicit
VisDelta expects tidy data and does not clean, pivot, join or repair it. Every visual encoding is declared. Presentation defaults such as radius, line width, margins and duration are supplied for you, but a field never becomes x, y, colour, size or detail unless you map it. Generated intermediate states may not invent or remap a colour channel.
Selection words have separate meanings
where() changes membership, highlight() changes emphasis and focus() moves
the camera. Focus never filters data; highlight never changes scales. See
where, highlight and focus.
Correspondence is evidence, not a route
Shared source records establish relationships between marks. They do not by themselves justify a particular motion. Missing or unsafe evidence keeps the chart's ordinary transition.
Reverse means the same frames backward
For a reversible route, progress p played forward and 1 − p played backward
show the same frame. Explicit directional easing is the documented exception.
Core knows no chart types
The generic runtime contains no branches for any built-in chart. Each chart module owns its builder, compiler, marks, axes, matching and motion, and the modules are peers. Adding a chart type should not require editing the runtime.
Keep the language small
One word for one concept. Builder syntax, emitted state-change actions, correspondence operations and transition phases stay visibly distinct.