The point of the picture is to imply that they all track the same thing: racial animosity. It ought to be obvious that this is misleading, as the voting pattern of the 2012 election is quite similar to those of 2004 and 2000 (and 1996 and 1992) where race was not an issue.
Republicans gained the South slowly, from the outside in. This happened as the hard-core racist Democrats became more and more of a social embarrassment.
As to why blacks are such a one-party block, there are many reasons: freebies from FDR and LBJ, principled-yet-unattractive opposition to the Civil Rights Act by conservatives (even though a higher percentage of Republicans than Democrats voted to pass it), and the rise of the racial grievances industry in the wake of the Civil Rights Act. The CRA did break the Democrat’s machine in the South, but the captive votes of welfare recipients kept them competitive nationally, as LBJ hoped.
Check out http://www.270towin.com/historical-presidential-elections/ to see ALL the elections. 1964 was a disaster politically and for the credibility of Republicans on civil rights, but as you can see from 1968 and 1976, the parties did not suddenly ‘switch sides.’ As I said, the hold of the ‘Good Ol’ Boys’ had to be pried away slowly.
Oh, and as you mention being young, I should mention that despite what you were taught in public school, both President Lincoln and the Rev. Martin Luther King, Jr. were Republicans. :)
There's one more reason: Communist penetration of Black churches. Communists originally attacked the Black church and Black preachers but at a certain point in the 1930's they switched their strategy and started infiltrating church youth groups. This combined with their penetration of seminaries means the majority of Black churches, even in the rural South, were under Communist influence for decades and eventually they switched to 100% social gospel. There are still a few fundamentalist Black churches left (never called "fundamentalist" since that label is reserved for "racist rednecks") but they still vote Democrat just like a few white "yellow dog Democrats" do. That and the fact that they churches have always tended to create J*sus in their own image leading to worshiping two separate "gxds" (whites worshiping "redneck J*sus," Blacks worshiping "J*sus X--Soul Brother Number One") which divides churches ethnically and keeps them from being true co-religionists.