Indeed, there's another feature I don't fully understand: I have a bgiWig file that contains reads of only one chromosome. I expected that Circster would display this one chromosome as one circle, but apparently Circster always draws a circle where all possible chromosomes of a genome are displayed. I think the usability would greatly increase if Circster only displayed those chromosomes that are actually represented in the coverage file. (Of course, I could zoom in, but if you're working with a chromosome that's very small in comparison (e.g. the Y chromosome) the circular representation is not really seen anymore as the region covered by the Y chromosome is so tiny compared to the autosomes).
Circster is really for genome-wide visualization, and the assumption is that you'll have data for many if not all chromosomes. If you have data for only a single chromosome, using Trackster (Galaxy's track browser) makes more sense; Trackster is also more developed and has more display options right now.
Let's say, then, that what you're proposing is a very advanced feature that could be implemented down the road.
Best, J.