Feature: Plot Math Functions

* [ x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## Motivation
A common use for Charts is to plot mathematical functions. Building in the ability to plot arbitrary functions would enable a whole class of apps, and could be a valuable resource to students.

## Proposed Implementation
A new chart type that takes an arbitrary function, and generates sufficient values to fill the visible area of the chart (plus a small overscan). Panning/zooming the chart should generate the additional entries and insert them at the start/end as appropriate.

Start by reviewing the repository's existing chart types and how visible-area changes, panning, and zooming are handled. Define the function input, sampling and overscan behavior, and how generated values are inserted; done means arbitrary functions render correctly and update when the visible range changes.

Written by the indexing model from the issue text.