Click here to see an animated videographic of this chart

Everyone knows that American politics has become more polarised over the past quarter-century. Now the effect can be shown visually—with troves of data, statistics and “force-directed graphing”, a technique to depict positions based on attraction-and-repulsion models adapted from physics.

The network maps shown here look at the degree to which senators vote the same way. Each node is a senator. Links represent instances when senators have voted similarly on substantive legislation on at least 100 occasions during the same congressional session. Their placement is determined algorithmically, based on their co-operation with other legislators—which has the effect of pushing more bipartisan ones to the centre.