In dygraphs 1.x, if you set a valueRange for the y-axis, zoomed in and then double-clicked to zoom out, then the chart would zoom out to the valueRange that you specified.
Starting with dygraphs 2, double-clicking will zoom out to the full range of the chart's data. This makes the x- and y-axes behave identically.
This demo shows you how to get the old behavior in dygraphs 2 using a plugin. View source to see how.