An Iter is an iterator function that returns count number of Points or an error.
A Point in a line chart.
Downsample `count` number of data points retrieved from the given iterator function to contain only `threshold` number of points while maintaining close visual similarity to the original data. The algorithm is called Largest-Triangle-Three-Buckets and is described in: https://skemman.is/bitstream/1946/15343/3/SS_MSthesis.pdf
This implementation grew out of https://github.com/dgryski/go-lttb to limit memory usage by leveraging iterators.