By the way, how were the teams selected for the graph? It looks like there were 2 outlier teams, one with 50 some wins and another with 110 or so wins that act as stabilizing points that have a big impact on the slope of the graph. A WAR should be a win, meaning that the slope should be 1.00, but it is 0.97 meaning that a WAR is only equivalent to .97 wins. That leaves .03 wins per war unaccounted for. Given that an average team produces 81 wins versus 52 for a replacement team, the average team is getting 29 WAR. .03 x 29 is 0.87 wins unaccounted for so the estimate that a replacement catcher may be 0.4 to 0.7 war lower than other positions would be within the .87 wins missed by war in the equation.
Edit, actually teams are winning .97 per WAR, not getting 1 win per .97 war so the average team is .87 wins below what they would have with a perfect correlation. Still, I'd like to see the equation with the two outliers removed.