Stacked bar graphs are tricky, particularly when you design more than one and you arrange them in a sequence: Only the bottom and upper portions are comparable to each other, as they sit on common baselines. However, there are cases when this graphic form is appropriate. See this elegant small multiple array just published by The New York Times.
What matters in this graphic is not to compare all parties, but to emphasize the hard-right ones, and then to compare them to all other parties as a whole. Therefore, I think that the decision of coloring all center-right and center-left parties identically makes sense: It's red versus white and gray.