2) a consequence of 1 in fact: if you zoom or move the graph your circle will remain in "old" position and scale, which is annoying.
This is how you draw a filled circle of radius R at (x,y) in the axis of your graph using "area" command:
Ang = 0:0.01:2*pi; %angle from 0 to 2pi with increment of 0.01 rad. CircX=R*cos(Ang); CircY=R*sin(Ang);
set(h,'FaceColor',[.7 0 0]) %colour of your circle in RGB, [0 0 0] black; [1 1 1] white set(h,'LineStyle','none')
% unfortunately you have to remove the line: because circle is not a function strictly speaking. % "area" command tries to fill area below your function which leaves a line-artefact connecting your circle with X axis.