A proportion is easiest to read against the whole of the interval from 0 to 1, so that is the range a plot of one shows by default. The range is only ever widened to include the data, never narrowed to the interval: a critical feeding level or a resource level above 1 is a real feature of the model and must stay visible.
proportion_ylim(ylim, log_y, values)A numeric vector of length two.
Numeric vector of length two, the limits the caller asked for.
Whether the y axis is logarithmic.
The values being plotted.
Only the ends of ylim that the caller left as NA are filled in, so an
explicit limit always wins. A logarithmic axis is left alone, having no place
for the 0.