You can change a static point to a movable point by clicking and long holding the icon next to the expression list. A style editor will pop up with different drag options.
Another way to create a movable point is to enter a point with a parameter for one or both coordinates. Click and drag the point around the graph to change the value of the parameter(s).
To make graphs more interactive, use parameters from your movable point in your expressions.
Explore this graph here: https://www.desmos.com/calculator/slkjzmm3ly