// draw 3D axes

void drawAxes(float axis_length) {
  stroke(#FF0000);
  line(0.0, 0.0, 0.0, axis_length, 0.0, 0.0); // x-axis
  stroke(#00FF00);
  line(0.0, 0.0, 0.0, 0.0, axis_length, 0.0); // y-axis
  stroke(#0000FF);
  line(0.0, 0.0, 0.0, 0.0, 0.0, axis_length); // z-axis  
  noStroke();
}