1. Select number area where to create the heatmap, click Format > Conditionnal > Color Scale
2. You can click Ok in the next dialog box unless you want to adjust color thresholds :
3. Here is the result of the example :
4. You can easily expand that to multiple columns as well :
I googled "heatmap LibreOffice" but did not found what I was expecting so I decided to blog about it.