I have a GUI that displays a graph and I want to have some help with a code that allows you to click a point and then another point and then it draws a line. I need some help with this because I don't know were to start.

Dear u can do this by seving both left buton click in 2 variabli and just use drawline funcrion for drawing a line

In the MouseEvent's you can do this to get the location of the click:

Event.getX();

Event.getY();

That would help you with the coordinates of where the click happened, and where you should draw the line.

ok but how is that info stored?

you can store it in a double:

double x1 = Event.getX();

double y2 = Event.getY();

I don't know how you are going to tell if the first points were clicked unless you use a boolean...Like that boolean is set to true if the frst points are clicked, and then set to false..If that boolean is set to false, y would then store to the points in the second set of coordinates:

double x2 = Event.getX();
double y2 = Event.getY();