The map was produced with what at that time was a cartographic innovation.. the use of a shaded color ramp to represent ranges in values.. in this case the number of slaves per county in the southern states.
Source: via Disunion
From the post:
The map reaffirmed the belief of many in the Union that secession was driven not by a notion of “state rights,” but by the defense of a labor system. A table at the lower edge of the map measured each state’s slave population, and contemporaries would have immediately noticed that this corresponded closely to the order of secession. South Carolina, which led the rebellion, was one of two states which enslaved a majority of its population, a fact starkly represented on the map.
The comment thread is interesting too... especially this argument.