Set minpressure from manually added cylinders

This includes minimum pressure from manually added cylinders to be taken
into account on scaling. Without this, manually added cylinders might
lead to pressure dropping below the Y axis 0 line (e.g. when first
"computerized" cylinder is 220->140 and second, manually added cylinder,
200->50 bar).

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>