Hi again,

before a year ago when we were doing the graphics class at college my professor said that the coordinate system is reversed in programming that means that the Y axe is positive when facing down and the X axe is negative when facing right , however when using the graphics class in c #, the X axe is not negative on the right side instead it is POSITIVE! i said to myself that i will learn this basics but somehow i can t find Anything on google about this and when doing some project i nearly always end up using some false coordinates and the main reason is because i am stil not sure how this coordinate system works in programming and by the way Why is it Reversed in programming ??

Pixels on the screen are counted from the top left(0,0) of the screen. This is the way video works I guess.
So X goes from 0 to the horizontal screen(right) resolution(positive direction)
Y goes from 0 to the vertical screen(bottom) resolution(negative direction)

finally, tnx, can you please suggest a site where i can look more on this , or anybody else?

Yes, what about DaniWeb? This is a snippet I posted here. Forget about the sawtooth, look for a Plot class. these days I would use the Chart component Available as of VS2010 I think.

Just my cent.

Additional to the excelent info from ddanbe, here after a couple of links:
Windows forms coordinates on MSDN (here)
Coordinates systems on Wikipedia (here)

Only to say that the values to define a drawing point in a screen are always in positive values referred to the top-most/left-most corner of the screen. As X axis increases, the point is moved to the right and as Y increases, the point is moved down.

Hope this helps