Heatmap

When to use

Heatmaps are good for:

Correlation matrices between many variables.
Activity data over a two-dimensional grid (day × hour, row × column).
Confusion matrices and cross-tabulations.

Use ax.Heatmap(matrix, chart.HeatmapStyle{...}) where matrix[row][col] is the scalar value for that cell.

Basic example

Pairwise correlation matrix for four business metrics — positive correlations in warm colours, the negative churn correlation in cool.

package main

import (
	"github.com/goplotlib/goplotlib/chart"
	"github.com/goplotlib/goplotlib/colormap"
	"github.com/goplotlib/goplotlib/plot"
)

func main() {
	// Pearson correlations: Revenue, Profit, Active Users, Churn Rate
	matrix := [][]float64{
		{ 1.00,  0.82,  0.65, -0.43},
		{ 0.82,  1.00,  0.51, -0.37},
		{ 0.65,  0.51,  1.00, -0.71},
		{-0.43, -0.37, -0.71,  1.00},
	}
	labels := []string{"Revenue", "Profit", "Users", "Churn"}

	fig := plot.New(plot.WithWidth(600), plot.WithHeight(480))
	ax := fig.AddAxes()
	ax.Heatmap(matrix, chart.HeatmapStyle{
		RowLabels:      labels,
		ColLabels:      labels,
		ColorMap:       colormap.RdBu,
		DivergingScale: true,
		CellLabels:     true,
	})
	ax.SetTitle("Metric Correlation Matrix")
}

Heatmap

Colourmaps

Import "github.com/goplotlib/goplotlib/colormap" and pass one of the built-in maps:

Constant	Description
`colormap.Viridis`	Perceptually uniform, blue → green → yellow
`colormap.Plasma`	Blue → purple → orange
`colormap.Greys`	White → black (sequential)
`colormap.Blues`	White → dark blue (sequential)
`colormap.RdBu`	Red → white → blue (diverging)

ax.Heatmap(matrix, chart.HeatmapStyle{
	ColorMap:       colormap.RdBu,
	DivergingScale: true,
})

Use DivergingScale: true with diverging maps to centre the colour scale at zero.

Cell labels

// Print the numeric value inside each cell
ax.Heatmap(matrix, chart.HeatmapStyle{CellLabels: true})

Text colour is automatically chosen as white or dark based on cell luminance.

Style reference

chart.HeatmapStyle fields:

Field	Type	Default	Description
`RowLabels`	`[]string`	index	Labels for each row (y-axis)
`ColLabels`	`[]string`	index	Labels for each column (x-axis)
`ColorMap`	`colormap.ColorMap`	`Viridis`	Colour map to use
`CellLabels`	`bool`	`false`	Print the numeric value inside each cell
`DivergingScale`	`bool`	`false`	Centre normalisation at zero (for diverging maps)

Bubble Chart Pie & Donut Chart