Handle edge case where measurements' std dev exceeds x-scale

[joverlee521]

First noted by @huddlej in review.

I only noticed a couple of edge case issues where the std dev could exceed the lower and upper bounds of the x scale. For example, in the preview app if I filtered the source to "nimr-sep-2011-2011-06-24" and the clade reference to "158N/189K", I get the following mean/std view:

And this is the raw view: