If you have the data in a form of X and Y data points, you can do something like this
x = randi([220,800],100,1); %100 random x data points between 220 and 800
y = randi([200,600],100,1); %100 random y data points betwen 200 and 600
myindx = y - 0.5*x >= 0; %index which is above a line from x1 = 400, y1 = 200,
%x2 = 800, y2 = 400. This gives a slope of 0.5 and y intercept of 0.
NewX = x(myindx); %x values only above your myindx line
NewY = y(myindx); %y values only above your myindx line
plot(x,y,'o',...
NewX,NewY,'o',...
[400,800],[200,400],'-k')
