I'd love to add non-uniform interpolation schemes of higher degree than linear to Interpolations.jl - the two main reasons for why it hasn't been done already are time (focus has been on reaching feature parity with Grid.jl, for which the package started out as a replacement effort) and knowledge (I don't know about good algorithms for it). The second one is easily solved if you can link to a few good papers on the subject and/or liberally licensed implementations - the first one is best fixed with a PR :)
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