Question

Just like hold on in Matlab.
I met this problem when i want to draw several lines in one picture.
The number of lines varies.

Answer 1

Mathematica has a lot of constructs allowing you to combine plots, graphics and images.
I’ll give you several examples. You can search the help system to find out more and understand the subtleties of the examples bellow:

Edit

If you want to read/write from/to files, the following code may serve you as a starting guide:

(*Create data*)
data = RandomReal[1, {8, 2}];
(*Put into a file*)
Export["c:\\test.tab", data, "Table"];
(*Read the file into another list*)
rdat = Import["c:\\test.tab", "Table"];
(*And Plot it like before*)
Graphics[{Line@#, Red, PointSize[Large], Point /@ #} &@rdat]