boolean fixed = false;
int saveX, saveY;

void setup() {
  size(500,400);
  fill(255,0,0);
}

void draw() {
  background(255);
  int x,y;
  if (fixed) {
    x = saveX;
    y = saveY;
  } else {
    x = mouseX;
    y = mouseY;
  }
  line(x, y, x+25, y-50);
  ellipse(x+25, y-50, 10, 10);
}

void mousePressed() {
  if (!fixed) {
    fixed = true;
    saveX = mouseX;
    saveY = mouseY;
  }
}

void keyPressed() {
  fixed = false;  // release the pin
}