Source and target color transparency for line and arc datasets

**Is your feature request related to a problem? Please describe.**
Currently the line and arc datasets allow you to specify differing start and end colours for the lat/long pairs, however it doesn't seem to be possible to specify an alpha/transparency value. Being able to scale transparency along the length of the line (e.g. alpha = 0.3 at the source and 1 at the target) would be a neat feature for indicating direction of an line or arc.

**Describe the solution you'd like**
The ability for the user to specify an RGBA or transparency value when setting the source or target color for an arc or line, such that the transparency graduates between the source and target values along the length of the line.

**Describe alternatives you've considered**
Nil.

**Additional context**
Nil.