Y-Axis Labels

Bar, histogram, stacked bar, stacked area, line, and live line charts share one Y-axis label pipeline in charts-core. Ticks are evenly spaced from the maximum at the top to the minimum at the bottom, each label is centered on its tick, and every label stays inside its column.

Pipeline

All files are in charts-core/src/commonMain/kotlin/io/github/hdcharts/charts/internal/common/axis/.

Ticks

yAxisTickCount shows AxisLabelStyle.maxCount ticks, or five when it is null, but never more than fit yAxisLabelMinSpacingPx apart over the plot height between the insets, and never fewer than two, the two ends of the range. Validation rejects a set maxCount below 2.

yAxisLabelMinSpacingPx is one label line plus the gap between X-axis labels:

minSpacing = fontSize × 1.2 × (1 + 0.5) = 1.8 × fontSize

Y labels are level, so one line of text plus the gap keeps neighbors apart. At the default 11sp size, ticks stay at least about 20dp apart. A 5-tick axis needs about 80dp of plot height; shorter plots show fewer ticks.

Bar and stacked bar charts decide how many bars fit before they know the plot height, so they estimate the Y-axis column at the full chart height. On a short plot that estimate has more ticks than the drawn axis, and its column can be a character wider or narrower. Only the number of bars that fit uses it; the plot is laid out with the drawn column.

buildNumericYAxisTicks spaces the ticks evenly over the plot height. Line charts inset the first and last tick so the line, points, and selection markers fit at the minimum and maximum. The other charts use no inset.

Each tick value is max × (1 − p) + min × p, where p runs from 0 at the top to 1 at the bottom. It stays finite for any finite range, including -Double.MAX_VALUE..Double.MAX_VALUE, where max − min overflows, and it returns max and min exactly at the two ends. When min == max, every tick is that value: blending a value with itself can round to the neighboring double, and a flat line at 0.375 with 11 labels would then show 0.37 among its 0.38 labels.

Formatting

Bar, histogram, line, and live line charts format ticks with their axisValueFormatter. Stacked bar and stacked area charts have no public formatter yet (see "Stacked charts have no public formatters" in Y-Axis Label Issues) and use defaultAxisValueFormatter, which matches the bar and line defaults:

• It rounds to two decimals with ChartValueFormatters.Default and drops a trailing .0: 12.345 prints 12.35 and 12.0 prints 12.
• It prints plain digits on every platform, so 12,500,000 prints 12500000, never 1.25E7.
• It prints values that round to zero as 0, and NaN, Infinity, and -Infinity as they are.

Placement

AxisYLabelsLayout places each label with placeYAxisLabel:

• The label is right-aligned 4dp from the right edge of the label column.
• It is centered on its tick, then moved back inside the column if it would stick out at the top or bottom. A label taller than the column starts at the column top.
• A tick more than 1 px above or below the column has no label. The slack covers a fractional plot height that is laid out in whole pixels.

Every chart builds its column with rememberNumericYAxisLayout. estimateYAxisLabelWidthPx sizes the column from the longest label, at 0.58 × font size per character, and yAxisLabelColumnWidthPx caps it at 40% of the chart width and rounds it to whole pixels. Longer labels are cut off. AXIS_LABEL_CHART_GAP (10dp, rounded to whole pixels) sits between the column and the plot, shrunk if needed so the plot keeps at least 1 px. Hidden labels take no ticks, width, or gap.

Tests

Brute-Force Checks

These functions were compared with plain definitions of their output. Every case matched.

buildNumericYAxisTicks_equalMinAndMax_keepsExactValueForEveryCount in AxisHelpersTest repeats the flat-range check for 0.375, which prints a wrong label without the flat-range rule.

Known Issues

Known issues and limits are listed in Y-Axis Label Issues, in the Known Issues section.