Make graph plot wait until error checking is done

if x1 > x2
   while Start > x2
   disp(['The Start value must be less than: ', num2str(x2)]);
   Start = input('Please, input a Start value of x: ')
   end
   while End < x1
   disp(['The End value must be greater than: ', num2str(x1)]);
   End = input('Please, input an End value of x: ')
   end
 elseif x2 >x1
   while Start > x1
   disp(['The Start value must be less than: ', num2str(x1)]);
   Start = input('Please, input a Start value of x: ')
   end
   while End < x2
   disp(['The End value must be greater than: ', num2str(x2)]);
   End = input('Please, input an End value of x: ')
   end
 end
            ^
            |
This bit is includes the plot axis etc.
            |
            v
end

How do i get the plot to wait until the first section is done??