Isochrons adjust with zoom

[johnzeringue]

@noahmclean, as you find time, I'd appreciate some help with the isochron math. While I've translated your Matlab code, I don't understand the math behind it well enough to extrapolate the information I need. I essentially need to solve for the isochrons that pass through the upper left and lower right corners of the graph.

With this, a fresh set of isochrons can be calculated for any plot bounds, and we can select them at varying intervals as the user zooms in or out.

[bowring]

Related to this request is a further explanation of the various flavors of axis that the evolution plot can use. @noahmclean has informed me that in addition to activity ratios, atom ratios, and delta234U (both of which are transformations of activity ratios) can apparently be used in any combination for the x and y axes. A more formal description of this is requested along with the label specifications for the axis in each case.. This will directly impinge on @johnzeringue's implementation of the solution you provide to his question above. At this point, having a good understanding of all the possibilities a user might want to invoke for an evolution plot would be great.