Quiver Demo for iOS

Personal finance apps show totals and categories, but they rarely reveal what changed and why. Which days broke the pattern? How are spending habits shifting week to week? Answering these questions requires outlier detection, percentage normalization, and downsampling.

This demo uses Quiver to build a spending dashboard entirely on-device. groupedData(by:using: .percentage) shows where money goes. downsample(factor:using:) converts 30 days into weekly summaries. outlierMask() flags the days that broke the pattern. All computation runs on Swift arrays — no server, no bridge, no third-party analytics SDK.

Run it

1. Clone this repo
2. Open in Xcode 26
3. Run on the iOS simulator

Screens

This Month — Total spend, daily average, daily spread, and month-over-month percentage change computed with sum(), mean(), standardDeviation(), and percentChange(lag:). Every headline number shows up with the spread next to it — the daily mean alone hides whether spending is steady or volatile.

Where It Goes — Donut chart of spending by category, powered by groupedData(by:using: .percentage) — one call aggregates and normalizes.

Weekly Breakdown — Bar chart of weekly spending totals computed by downsample(factor:using:) — one call chunks 30 days into 5 weeks.

Unusual Days — Scatter chart highlighting outlier spending days detected by outlierMask() with dollar annotations on each flagged day. The subtitle shows the live derivation — mean, standard deviation, and the actual dollar cutoff — so the 1.5-threshold becomes a visible computation rather than a black box.

Quiver APIs used

• sum() / mean() — monthly total and daily average
• standardDeviation() / standardError() — daily spread and confidence in the mean
• percentChange(lag:) — month-over-month spending change
• groupedData(by:using: .percentage) — category shares normalized to 100%
• downsample(factor:using:) — chunk daily data into weekly summaries
• outlierMask() — z-score flagging of unusual spending days
• maskedWithIndices(by:) — extract flagged days with their positions

Learn more

• Quiver — the framework
• Quiver Cookbook — 41 interactive recipes
• Quiver Documentation — API reference and conceptual guides
• Swift Algorithms & Data Structures — the companion book