Here's a function that will plot std ellipses where the vertical distance to the center is 1 std of y and the horizontal distance to the center is 1 std for x.
function h = plotEllipses(cnt,rads,axh)
% cnt is the [x,y] coordinate of the center (row or column index).
% rads is the [horizontal, vertical] "radius" of the ellipses (row or column index).
% axh is the axis handle (if missing or empty, gca will be used)
% h is the object handle to the plotted rectangle.
% The easiest approach IMO is to plot a rectangle with rounded edges.
% EXAMPLE
% center = [1, 2]; %[x,y] center (mean)
% stdev = [1.2, 0.5]; %[x,y] standard dev.
% h = plotEllipses(center, stdev)
% axis equal
% get axis handle if needed
if nargin < 3 || isempty(axh)
axh = gca();
end
% Compute the lower, left corner of the rectangle.
llc = cnt(:)-rads(:);
% Compute the width and height
wh = rads(:)*2;
% Draw rectangle
h = rectangle(axh,'Position',[llc(:).',wh(:).'],'Curvature',[1,1]);
end
Here's a demo that plots random dots from two different distributions and adds colored std ellipses.
% Create data
d1 = randn(2,500)+1;
d2 = (randn(2,500)-2) .*[1;2]; %greater spread along y-axis.
% Compute mean and std
mean1 = mean(d1,2);
std1 = std(d1,[],2);
mean2 = mean(d2,2);
std2 = std(d2,[],2);
% Plot data
figure()
plot(d1(1,:),d1(2,:),'b.')
hold on
plot(d2(1,:),d2(2,:),'r.')
axis equal %this is important so circles appear as circles
grid on
% Plot ellipses, then change their color and other properties
h = plotEllipses(mean1,std1);
h.FaceColor = [0 0 1 .3]; %4th value is undocumented: transparency
h.EdgeColor = 'b';
h.LineWidth = 2;
h = plotEllipses(mean2,std2);
h.FaceColor = [1 0 0 .3]; %4th value is undocumented: transparency
h.EdgeColor = 'r';
h.LineWidth = 2;
