I believe the best option is this:

DygraphOptions has three methods:

get(name)
getForSeries(name, series)
getForAxis(name, axis)

Just expand them:

get(name, [includeDefault])
getForSeries(name, series, [includeDefault])
getForAxis(name, axis, [includeDefault])

In the xAxisHeight case, just call get('xAxisHeight', false)