Center time-interval names to the plotted part of the time interval

[mcranium]

I noticed that there is much space in some scales that is not used efficiently. For example, in the "Stack multiple scales (e.g. periods, epochs, stages)" example on the readme file the word "Cretaceous" is positioned at the center of the Cretaceous. Thus, the word is cut off in the plot. Could the text element not be centered to the visible part of the time interval?

[willgearty]

My original idea here was to put this responsibility on the user. You can get any of the timescale data, modify it to your liking, then supply it to the function.

However, I think this would be possible and makes sense as an option that many users might want, so I'll look into it when I get a chance.

[willgearty]

This should be fairly simple by just modifying the dat object such that the ends of the edge time periods are the axis limits. We just need to make sure we don't make all of the hidden periods show up.

[willgearty]

This has been implemented in ce45a54. Let me know if you have any trouble using the new argument, @mcranium.