add symlog.xOffset() so symlog can be centered away from x=0

[kussmaul]

I would like to have symlog.xOffset() center the function at a given x-value, which would be subtracted from each x in the forward equation, and added to the inverse equation. Currently, the Symlog scale is centered at x=0, and symlog.constant() adjusts the slope near the origin. I think xOffset() would be useful, backwards compatible (defaults to 0), and not difficult to add to the current code.

Context: My data ranges over days or months, but has minute-level features. I am working on a swimlane chart (inspired by this example) which shows the full range on a mini-chart and a selected subrange on a main-chart. Currently, it is difficult to select a subrange to see minute-level features. Thus, I would like to to use a symlog scale on the mini-chart, centered on the subrange (not on 0); the main-chart would have a linear scale.

Thank you for your consideration and feedback.