1. range or scale
on/within the spectrum, on/across the social/political spectrum
A spectrum refers to a range or a scale. When we say something is 'across the spectrum' we mean that it spans a number of areas or encompasses many things or variables. This is used often in business contexts or to talk about social groups or class: "People across the socio-economic spectrum went out in force to protest the Government's decision". While using this phrase you might want to mention which spectrum you are referring to.