Scale bar option

[whelena]

Add optional param to SRCGrob to use scale bars instead of y-axes.

Using a scale bar allows users to estimate the distance for each edge, even when the edges are not vertical. It may not provide perfect precision, but it avoids the misleading visual representation caused by a y-axis, which can distort the perceived distances between nodes. Additionally, when using 2 edges, this allows us to indicate which edge corresponds to which value (blue vs. dashed black line below) without using figure captions.