chart: { history: { enabled: true } }.
Every data update, zoom, and point selection commits a checkpoint. Press
Randomize a few times, then step back with Undo / Redo (or press
Cmd/Ctrl + Z and Shift + Cmd/Ctrl + Z). Toggling a
series in the legend and zooming also create checkpoints. The buttons enable
from the historyChange event via chart.history.state().