packing non-overlapping transcripts on the same y

[gpertea]

I realize this involves some complex design/implementation changes and it is a feature more suitable for a full genome browser which is not the goal of this package.

For cases where many smaller non-overlapping transfrags are generated during transcript assembly, it would be very helpful to be able to pack some of those on the same horizontal line/slot, thus reducing the overall height of the plot needed to show a rather large number of transcripts whenever such partial transcript fragments are present.

Of course this involves taking the transcript labels off the y axis and placing them next to each transcript (above or below the left-most exon, or centered?). This is clearly a major change in ggtranscript's current design and I do not expect to be implemented or even paid attention to, at the moment.