In the general form where you might have a number of different objects plotted in the same axes, and the data is not necessarily starting from zero.
factor = 1/20;
ax = gca();
all_y = {findobj(ax, '-property', 'YData').YData};
maxy = max(cellfun(@max, all_y));
miny = min(cellfun(@min, all_y));
dylim = (maxy - miny)*factor
ylim( [miny - dylim, maxy + dylim]);
In practice a lot of the time this can be replaced entirely by something like
ylim([0 max(y)*1.05])
where y is your vector of y values when you drew a single line using plot(x,y)
Note: the above code will fail if the data contains infinite values.
