3D function plotter and heat map for functions of two variables

For a function f(x,y) function values are determined over an equidistant point grid of the xy plane and plotted in the direction of the z axis.
The resulting surface is displayed in a 3D function diagram. Alternatively the data can also be displayed as a 2D heat map.
Because linear interpolation is used to determine the isolines, the entire area is covered by triangular surfaces supported on a grid of approximately 900 points.
The 3D presentation can be rotated by dragging the mouse.

Examples
x₁	x₂	y₁	y₂	3D-view
f(x,y)

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