There are two modes for creating parabolas
Two Point mode: here only two types are possible: Up and Down, e.g of the form: y = a x2 + bx + c
Focus and Directrix mode: This uses three points.
Select the mode by picking from the shape menu. See the two parabola modes at the bottom:
For the two-point mode (on left), see the "1" and "2":
first, as point "1", enter the vertex, then the orientation will be determined based on how you pick the second point "2".
y = x2
"A" is the first point, and "B" the second. To get the mirror, or upside-down version of this parabola, you should pick B as (-1,-1)
You can also put the mouse over the points and drag them to reconfigure the parabola.
As three points are generally required to completely specify a parabola this simple mode is provided for quick use, and it avoids the need to determine the focus and directrix.
But for the more general case, let's consider the example of
x = 4 y2
We will use the other parabola object:
Note points "1", "2" and "3" in the grey shaded selection above.
The first point, or focus is A: (1,0)
and directrix: the line x=-1, so we will pick points B: (-1,0) and C: (-1,-1)