Hello, this is an unexpected behaviour.
I have seen other similar problems with axis equal, but I am not sure it is related.

close
fplot(@sin,[-pi, pi]);
axis([-pi pi -1 1]*1.1)

after the plot shows up, make zoom on some region and finally
run 'axis equal'. The plot disappears leaving an empty axis.
By doing some pan or zoom in/out in the figure the plot might reappear.

This happens with qt graphics toolkit.

This does not happen however if one does

set(gca,'dataaspectratio',[1 1 1])

instead of axis equal.