extent difficult to compute

[trebor]

the extent() property nicely bounds the verticals for the chart, but it doesn't account for labels and paths which happen to appear outside of the area bounded by those verticals. as a result i need to manually set an extent after i've seen a rending to know how much padding i'm going to need to properly render the chart without obscuring the labels and paths which appear outside the extent. it would nice to have a way to bound the chart which included these outlying labels and paths so i can ensure my entire chart is plotted inside my target rectangle.