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04 — Bar Chart

A vertical bar chart: one bar per category, category labels on the X axis, the value printed above each bar, and a Y axis that always starts at zero. Adding a second series groups the bars side by side within each category.

Bar chart of the first ten Fibonacci numbers — Single series with value labels

Grouped bar chart of quarterly sales for two years — Two series grouped per category

Source on Forgejo

The code

chart := gogal.NewBarChart(
    gogal.WithTitle("Fibonacci (n=10)"),
    gogal.WithSize(500, 220),
    gogal.WithYFormat("%.0f"),
)
chart.AddCategories("Fibonacci",
    []string{"F1", "F2", "F3", "F4", "F5", "F6", "F7", "F8", "F9", "F10"},
    []float64{1, 1, 2, 3, 5, 8, 13, 21, 34, 55})
svg, _ := chart.RenderString()

AddCategories(name, categories, values) takes parallel slices. The categories become the X-axis labels; call it once per series to get grouped bars.

Baseline at zero. Unlike line charts, the Y scale always includes 0 so bar heights compare honestly. Negative values hang below the baseline. A little headroom is reserved above the tallest bar for its value label.

WithValueLabels(false) turns off the value printed above each bar. Hovering a bar dims it slightly — pure CSS, no JavaScript.

Sparkline bars. NewBarChart(gogal.WithVariant(gogal.Sparkline)) renders a bare 100×20 bar strip with no axes, for inline use in tables.

Full source on Forgejo

Running it

task example:04

Outputs SVG to stdout and serves at http://localhost:1343.

API reference

Function	Purpose
`NewBarChart(opts...)`	Vertical bar chart
`AddCategories(name, categories, values)`	One series; repeat for grouped bars
`WithValueLabels(on)`	Print each value above its bar (default on)
`WithYFormat(fmt)`	Y-axis tick format (Printf)

Horizontal and stacked bars are not yet implemented — see the ROADMAP.

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