Well this is worth printing out and analyzing with your class. It’s an in-depth (to say the least) look at the trends of Congress and how they respond to who is elected president. What could be more topical? This infographic is from the must-follow XKCD and is one of the most elaborate things I’ve ever seen come out of that site.
The infographic below is dense and requires a lot of time to analyze and understand. That seems to be the trend with infographics these days. In any case, pay special attention to the legend(s) on the top right side of the graphic. It outlines what the various strands mean, what the color coding indicates, and how the visualization was built.
Then start focusing on the actual colored strands. I recommend starting at the bottom with George Washington. Then pay special attention to the little blurbs along each strand. They outline why there are outliers, what that means, and why.