use gluPerspective() instead of glFrustum()
rewrite generation of stairs/interval and more

    add Point2D class
    intervals are generated w/ Point2D and w/o crossing
    init_data on button press
    use glDrawArrays() to draw data
    add util.py to flat points and segments coordinates (for glDrawArrays())
    add test example (x min_x max_x format atm)
    add function to start reading file format