VisualChannel doesn't deal with a mapping to an optional property

Currently Chart's visualChannel setup doesn't handle optional values within KeyPaths. For example, `\.something` that's a `Double` is fine, but one that's `Double?` isn't. 

While messing around with CodableCSV, I found it useful to want to reference an optional property, with the goal of skipping the data if it returned nil, but that seems... to not be working at the moment.