For example, you can click the highest data points for CO2 levels and horizontal and vertical lines will mark the point in the same color as the graph that was clicked:
Then, if you rescale the x axis, say to zoom in on the Holocene, you can see the y values of the data points you flagged from 323 k and 414 k years ago:
As always, I continue to take requests for features. My next big change (I hope) will be allowing multiple views of the x-axis timeline, so you can directly compare the shape of graphs between different time periods. My data viewer is available here: Epica-Vostok viewer.