GrChartWaterfall

• movement from balance to balance — subscriptions, liabilities, a warehouse: not only every change is visible but also how the end of the month came out of its beginning;
• checking whether the balance adds up — kind: 'total' steps put the real values at the start and at the end, and a discrepancy with the sum of the movements becomes visible to the eye rather than in one’s head;
• breaking down the delta between two numbers — “revenue grew by 12 %” and the four reasons for that growth read as one story;
• increases and decreases mixed together — the colour follows the sign, and the direction of every step is visible before the labels are read;
• there are few steps — up to fifteen or twenty; beyond that the labels of the axis run into one another, and the bridge is worth folding down to large items.

kind: 'delta' (the default) is a change from the current accumulation, and kind: 'total' an absolute value: the column is drawn from zero, and the accumulation is reset to it. That is what makes it possible to put “At the start” and “At the end” as real numbers from the backend: if the bridge does not add up, the last column will not match the top of the one before it, and the error in the data will be visible.

A connector to such a step is not drawn. It does not continue the accumulation but declares it, and a “from here to here” line would lie about the succession.

A bridge is a single series, and the palette of series distinguishes nothing here: what has to be distinguished is an increase (--gr-success) from a decrease (--gr-danger). The total and the declared accumulation are coloured neutrally (--gr-chart-1) — that is not a movement. An explicit color on a step is stronger than any of those rules.

“There was no movement” is a fact rather than an absence of data. A column of zero height would disappear from the drawing, and the reader would see a story in which such a step does not exist at all. Instead a rule of --gr-chart-waterfall-zero-step thickness stands at the level of the accumulation.

The hidden table gives three numbers per step: the change, the accumulation before and the accumulation after. From a single delta the bridge cannot be reconstructed — the height of a column is set precisely by the accumulation, and a reader without sight is obliged to see the same as a sighted one sees from the position of the band.

Two “Adjustments” in a row in a bridge are an ordinary matter, and normalisation collapses identical categories inside a series. The index of the step therefore goes inside the frame, and the real labels arrive on the axis separately: otherwise there would be fewer positions than steps, and the cursor and the keyboard would travel over the wrong data.

The bridge does not compute what its steps consist of — they are given by the consumer. It does not group small steps into “Other” either: only the caller knows what counts as small. There are no nested (two-level) bridges.