plot different x ranges

Here is one way:

% Some made-up data
x = 1:0.25:10;
y = x + 0.5*rand(size(x));

% Identify the x ranges
redIndex  = x <  5;
blueIndex = x >= 5;

% Plot
figure
hold on
h = plot(x(redIndex),y(redIndex),  'r.', x(blueIndex),y(blueIndex),'b.');
set(h,'MarkerSize',24) % Don't really need this, but wanted the markers to really show