Bear in mind though that the heatmap script will produce a scan that is auto scaled. Which makes comparison with later scans difficult.
The range used is shown in the output from the heatmap command as the z parameter
It finds the highest and lowest figures in the csv and uses them.
eg from a scan I did:
./heatmap.py scan.csv scan.png
loading
x: 4576, y: 60, z: (-37.070000, 4.400000)
You can force as fixed scale to make future comparison easier.
I suggest using something like -40 to 10 as a fixed scale so:
./heatmap.py --db -40 10 scan.csv scan.png