I have made a little program to draw a line without the line(); funktion.

void setup(){
 size(500, 500, P2D);
 background(255);
 stroke(0);
 frameRate(5000);
}
void draw(){
float s;
 float x1 = mouseX;
 float y1 = mouseY;
 float x2 = width/2;
 float y2 = height/2;
 float px = x1;
 float py = y1;
 float c = 0;
 float g = 0;
 if(x2>x1){
   c = x2-x1;
   px = x1;
 }
 if(x1>x2){
   c = x1-x2;
   px = x2;
 }
 if(y2>y1){
   g = y2-y1;
   py = y1;
 }
 if(y1>y2){
   g = y1-y2;
   py = y2;
 }
 float x0;
 point(x1,y1);
 point(x2,y2);
 s=(y2-y1)/(x2-x1);
 x0=y2-(x2*s);
 println(x0);  
 for(int b = 0; b<c; b++){
 px++;
 py=s*px+x0;
 point(px, py);
 }
 if(x2>x1){
   c = x2-x1;
   px = x1;
 }
 if(x1>x2){
   c = x1-x2;
   px = x2;
 }
 if(y2>y1){
   g = y2-y1;
   py = y1;
 }
 if(y1>y2){
   g = y1-y2;
   py = y2;
 }
 for(int b = 0; b<g; b++){
 py++;
 px=(py-x0)/s;
 point(px, py);  
 }
 println(frameRate);
}
void mousePressed(){
 background(255);
}

perhaps someone of you knew how I can make it better.