# Flow Charts

> Composition and flow visualization - Sankey diagrams for cash flow and customer movement, marimekko for two-dimensional revenue mix, stacked versus 100% stacked rules, and where treemaps and pies genuinely apply. Trigger on "Sankey", "flow diagram", "where does the money go", "customer movement", "marimekko", "mekko", "revenue mix", "stacked bar", "100% stacked", "treemap", "composition".

- Skill: `lukehle/flow-charts` (Agent Skill)
- Install (CLI): `npx skillmds@latest add lukehle/flow-charts`
- Raw SKILL.md: https://api.skillmd.com/api/skills/lukehle/flow-charts/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Finance & Business
- Author: Lukehle (https://skillmd.com/u/lukehle)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/lukehle/flow-charts

---


# Flow and composition charts

Two questions that ordinary bar charts answer badly: **where did the money move** and **how does the
mix break down across two dimensions at once**.

---

## Sankey — movement between states

Bands whose width encodes magnitude, flowing left to right between nodes.

### The two that earn their place in finance

**Cash flow.** Sources on the left, uses on the right.

```
Collections ─────┐
                 ├──► Cash ──┬──► Payroll
Financing ───────┘           ├──► Vendors
                             ├──► Capex
                             └──► Ending balance
```

Instantly answers "where did the money go" in a way a table of the same numbers does not.

**Customer movement between periods.** The single best chart for retention dynamics:

```
Start-of-period ──┬──► Retained (same tier)
                  ├──► Upgraded
                  ├──► Downgraded
                  └──► Churned
New              ─────► End-of-period
```

Shows all four ARR-bridge components *and* their relative magnitudes and their destinations at once.
Where the bridge shows net movement, the Sankey shows the flows behind it.

### Rules

- **Widths must be exactly proportional and must conserve.** Inflow to a node equals outflow. If they
  do not, the diagram is wrong — the same footing discipline as `bridge-charts`.
- **Cap at ~15 nodes.** Beyond that it becomes a hairball. Group small flows into "Other" — and per
  the usual rule, "Other" must be smaller than the smallest named flow.
- **Order nodes to minimize crossings.** A Sankey with many crossing bands is unreadable regardless of
  correctness.
- **Label bands with values**, not just names. Width comparison is approximate; the reader needs the
  number.
- **Colour by source or by category**, consistently — never by band, or the eye tries to find meaning
  in it.
- Do not use a Sankey for a simple two-level split. That is a bar chart.

There is no Sankey primitive in `chartkit`. Compute node positions and band paths yourself: node
height is proportional to throughput, and bands are cubic Béziers between the two node edges.

---

## Marimekko — two dimensions at once

Column width encodes one measure (segment size), column height splits by another (mix within
segment). Area therefore encodes the product.

```
        │ Enterprise │ Mid-mkt │SMB│
        ├────────────┼─────────┼───┤
Platform│            │         │   │
        ├────────────┼─────────┤   │
Services│            │         ├───┤
        ├────────────┴─────────┴───┤
```

Excellent for "which segment is big, and what do they buy". A grouped bar chart cannot show both.

Rules:
- **Label both dimensions clearly.** A marimekko is unreadable without knowing what width means, and
  readers do not assume it means anything.
- **Show the width values** along the top — segment totals.
- **Limit to about 5 columns and 5 rows.** It degrades fast.
- **Only for parts of a whole.** If the columns are not a complete partition, the widths lie.
- Accept that precise comparison of non-adjacent cells is impossible; this chart is for the pattern.

---

## Stacked bars and areas — and their real limitation

The limitation people forget:

> **Only the bottom band sits on a common baseline. Every band above it is measured from a moving
> start, so middle bands cannot be compared reliably across categories.**

Consequences:

- **Put the series you most want compared at the bottom.** That is a design decision, not a default.
- **Two or three bands** is where stacking works well. Beyond about five, switch to small multiples
  or a line per series.
- **Consistent order across every bar**, always. Reordering by size per bar destroys comparability.
- **Never use stacked bars to compare a middle series across categories.** That is what small
  multiples are for.

### Stacked vs 100% stacked

| Use | When |
|---|---|
| **Stacked** | Both the total and the composition matter — total is the outline |
| **100% stacked** | Only the mix matters and totals differ wildly |

**100% stacked hides the total entirely.** A segment can be growing in absolute terms while its share
falls; on a 100% chart it looks like decline. If the total matters at all, show it — a total line
above the chart, or a companion chart.

---

## Treemap

Nested rectangles, area encoding value. Good for **hierarchical composition at one point in time** —
spend by department then category, revenue by region then product.

- **Never for change over time.** Treemaps show a snapshot; a reader cannot compare two treemaps
  reliably.
- **Area is judged poorly** — people underestimate large areas. Precise comparison is not available;
  label values.
- **Two levels maximum.** Deeper nesting is unreadable at any realistic size.
- A ranked bar chart beats a treemap for a flat list. Use a treemap only when the hierarchy itself is
  the point.

---

## Pie — the narrow legitimate case

Genuinely fine for **two or three parts of a whole**, where the split is roughly even and precision
is not needed. "Recurring vs non-recurring revenue" as a two-slice pie is clear.

Beyond three slices, or with a dominant slice and a long tail, use sorted horizontal bars. Angle
comparison is unreliable and ordering is invisible.

**Never a donut with a number in the middle.** The reader reads the number; the ring is decoration.
Show the number.

---

## Choosing among them

| Question | Form |
|---|---|
| Where did the money move? | Sankey |
| How did customers move between states? | Sankey |
| Which segment is big and what do they buy? | Marimekko |
| How does composition change over time? | Stacked area (few bands) or small multiples |
| What is the mix right now? | Sorted bars, or a 2-3 slice pie |
| How does spend nest by department and category? | Treemap (2 levels) |
| Net change in a total? | **Bridge** — see `bridge-charts` |

That last row matters: composition charts are frequently reached for when the real question is *net
change*, which is a bridge.

---

## Related skills

- `bridge-charts` — the net-change alternative, and the same conservation discipline
- `cohort-charts` — layer cake, the stacked-area cousin
- `chart-selection` — the wider decision
- `svg-charting` — building band paths
- `ui-antipatterns` — pies, treemaps and stacking failures