Better handling of "autoscale": the min and max are almost always taken into account (this is usefull to set fixed points, like the snow line, the sea level, etc) and the autoscale bool is used to tweak a given gradient for better display in autoscale (ie: removing the snow line in autoscale). The client is then responsible for provided the relevant min and max that he wants to use (constant -> no autoscale or variable -> autoscale).