Rendering and Animation Issues

Known issues and limits of the Rendering and Animation page in Chart Internals.

Need Verification

Frame cost of large charts is not measured

The issues under Confirmed follow from the code, but none of them has been timed on a device. Before choosing which to fix first, measure frame times for an expanded line chart, a live line window, an expanded stacked area chart, and a collapsed histogram at 10,000, 100,000, and 1,000,000 items.

Confirmed

Stacked area charts stay busy after a swipe on Android and iOS

After a swipe on an expanded stacked area chart, Compose never goes idle on Android and iOS. A test that waits for idle after the swipe hangs until its timeout, and a tap after the swipe does not select a new point in time. Line charts settle within about a second. JVM tests settle, so the cause is likely the platform fling or overscroll.

Confirmed by stackedAreaChart_scrollThenTap_changesSelectedLabelAtSameViewportX, which timed out on the iOS simulator, and a rewrite that waited for idle and hung on an API 35 Nexus 6 emulator. The test was removed until this is fixed.

Options:

• Find what keeps the chart busy after a swipe, starting from the scroll wiring in StackedAreaChartContent, and add the scroll-then-tap test back.

Line charts rebuild every point on every frame

During the reveal, a morph, or a live shift, every frame copies every value of every series, builds a new path through every point, and, for bezier lines, allocates an Offset per point. The expanded view also draws points scrolled off screen. Live line windows and expanded line charts are not compacted, so the cost grows with the item count.

Confirmed by the code: drawChartPath in LineChartDrawing.kt, and the canvas in LineChartContent, which maps every Animatable to a new list per series on each draw.

Options:

• Keep the drawn values in a reused FloatArray.
• Draw only the points in the visible range, plus one on each side.

One animation value per point or bar

Line, live line, and stacked area charts keep one Animatable per point of every series; bar, histogram, and stacked bar charts keep one per bar. A morph launches one coroutine per point, and a live shift calls snapTo on every point in turn: a two-series window of 100,000 points makes 200,000 calls per update. A histogram, which never compacts, holds one Animatable per bin, so 1,000,000 bins make 1,000,000 of them and a coroutine for each bin that changes.

Confirmed by the code: animatedValues in LineChartContent, StackedAreaChart, and StackedBarChart, and rememberBarChartAnimatedValues in BarChartAnimation.kt.

Options:

• Animate one progress value per chart, and blend the old and new values in the draw.

Stacked area builds every point before culling

Each draw copies every Animatable of every series, allocates a zero baseline as long as the data, and builds an Offset for every point in buildSeriesPoints, then keeps only the visible range with subList. Culling saves path segments, not the work over every point of every series.

Confirmed by the code: the canvas in StackedAreaChart and buildSeriesPoints.

Options:

• Build points only for the visible range, plus one on each side.

Wide expanded charts show an error instead of drawing

The expanded view sizes its canvas to every item, and Compose constraints cannot hold more than about 262,000 px. Past that, the chart shows "Chart exceeds layout limits" instead of drawing: an expanded line or stacked area chart at 21,847 points, or 5,463 at 4× zoom, and a bar or stacked bar chart with default bar sizes at 4,994 bars on a 2.625 density screen.

Confirmed by ChartCanvasLimitsTest and by the *_expandedPastLayoutLimits_displaysErrorInsteadOfCrashing tests in charts-line, charts-stacked-area, and charts-stacked-bar.

Options:

• Draw on a viewport-sized canvas and offset the drawing by the scroll position, so the canvas no longer grows with the item count. Positions then need to count from the first visible item: past about 8.4 million px, a Float position can no longer hold fractions of a pixel, and past about 16.8 million px it skips every other pixel.