To understand the mechanism under the hood, and to see the open source code, check here.
Points:
Zoom:
Export
Every setting lives in the URL, so the link restores this exact drawing.
Editing
Click empty canvas to add a point. Click a point to select it, drag to move it.
Shift+drag or scroll over a point changes its height. Alt+click or right click deletes it.
Scroll to zoom, drag empty canvas to pan.
On touch screens: one finger pans, two fingers pinch to zoom, and a tap selects the nearest point.
Edit or delete the selected point from the panel above — touch never moves, adds or deletes directly.