Parcoords - rgba not reflecting alpha

[MrHejtmar]

Issue

Currently when setting the line colors for parcoords, it is impossible to modify the line transparency using RGBA colorscale and set alpha to zero. It is reflected in the colorbar, but not with lines themselves.

As seen in the example, lines should change color from red - fully opaque, to red - fully transparent, according to alpha. However, the line color only reflects RGB properties and lines are red - fully opaque, yet the colorbar scale is shown correctly.

Example

https://codepen.io/anon/pen/ydYxbx

[etpinard]

That's correct - we don't support the alpha channel in parcoords colorscales at the moment.

related #3917