Removed all forced unwrap & support for empty data sets

[ekscrypto]

Should fix several bug reports about unwrapping nil values

[ekscrypto]

In commit 8311a64 I added support for empty data sets and 1-item data sets; several assumptions about minimum data set sizes like data.count - 1 were sprinkled here and there causing CGFloat.nan and to be generated in the linear scales then later creating crashes during rendering. While I attempted to maintain the current behaviour as much as possible, proper testing should be done to ensure the library is working for all supported scenarios.