// Variables that store the last mouse pressed position.
int lastX;
int lastY;

void setup() {
  size(500, 300);
}

void draw() { /* must exist */ }

// Draw a line from the last mouse position 
// to the current position.
void mousePressed() {
  line(lastX, lastY, mouseX, mouseY);
  lastX = mouseX;
  lastY = mouseY;
}