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.