MYSQL data + PHP to FLOTgraphing

Question

Hi guys, im quite new to mysql and flot graphing, but i get the general idea. This is my scenario: I receive data from a device, in which i put into mysql database. am i wrong in saying that the new data will replace the existing data in the database? i then need to plot that on a graph, how do i get(store) the old values so i can put in the data in this line?

 $(function () {
    var d4 = [[36,37],[50,51],null,[23,24],[18,17]];
    $.plot($("#placeholder"), [d4]);
  });

if not, i'll only be getting the current data... and that doesnt give me a line.. it'll give me datapoints haha Thanks for your help!