feature: heatmap

[johnfercher]

Description

Related Issue

Checklist

check with "x", ONLY IF APPLIED to your change

• All methods associated with structs has func (<first letter of struct> *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Followed the unit test when,should naming pattern.
• All mocks created with m := mocks.NewConstructor(t).
• All mocks using m.EXPECT().MethodName() method to mock methods.
• Updated docs/doc.go and docs/*
• Updated example_test.go.
• Updated README.md
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint

[codecov]

Codecov Report

Attention: Patch coverage is 12.82051% with 102 lines in your changes missing coverage. Please review.

Project coverage is 84.36%. Comparing base (4207ca1) to head (e9b1105).

Files with missing lines	Patch %	Lines
internal/providers/gofpdf/heatmap.go	7.23%	77 Missing ⚠️
internal/providers/gofpdf/chart.go	25.00%	15 Missing ⚠️
pkg/props/heatmap.go	0.00%	8 Missing ⚠️
internal/providers/gofpdf/provider.go	33.34%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
- Coverage   87.96%   84.36%   -3.60%     
==========================================
  Files          61       64       +3     
  Lines        2324     2441     +117     
==========================================
+ Hits         2044     2059      +15     
- Misses        245      347     +102     
  Partials       35       35              

Flag	Coverage Δ
unittests	84.36% <12.83%> (-3.60%)	⬇️

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