The load/length graph allows you to visualise the working positions of the spring, relative to various limits. In the example below, the load that corresponds to the unprestressed limit is visible at around 265 N. The two operating positions are visible, and the coloured parts of the line represent the allowable working lengths. The section in orange shows where the spring is compressed below the minimum dynamic working length (so this may be permissible, depending on the application), and the section in red represents compression below the minimum static working length. So you can see at a glance that this spring will be a prestressed design, and shorten if it is compressed to solid, but should be perfectly safe.