Have gnuplot plot this function:

a(x-h)² + k = 0

Try plotting your own graphs with Gnuplot.

Graph Type:
2D
3D Surface
3D Contour... Type of contour: base surface both
Animate your 3D graph:    standard enhanced

Optional Preferences:
Show grid lines

Use default range
Enter your own range (below)

Range for x: Low High

Range for y: Low High

Range for z: Low High

Remember, for two-dimensional graphs, the function is y=f(x).
For three-dimensional graphs, the function is z=f(x,y).