[twic]

The axis on the left is for the blue party rounds line, and the axis on the right is for the red total rounds line.

I assume this is because party rounds have never been more than 3.7% of total rounds, so showing them on the same scale would have left the party rounds line rather hard to glean anything from.

A better approach would probably have been to express the counts as ratios to their 2005 values; they could then have been presented on the same scale.