X, Y, Z Functions

It is possible to adjust position of grid points according to entered function(s). Function should follow rules of Ruby language syntax and may contain the following variables:

x - distance from grid origin to a grid point along x-axis
y - distance from grid origin to a grid point along y-axis
z - distance from grid origin to a grid point along z-axis
x_step
y_step
z_step
x_size
y_size
z_size

Here is a tiny tutorial, which may help to figure out how X, Y, Z functions work:

open "Grid" tool dialog
choose rectangular grid type, and set the following grid dimensions:
- width: 20000mm
- depth: 20000mm
- height: 3000mm
- X Step: 1000mm
- Y Step: 1000mm
- Z Step: 12000mm
refer to a screen-shot below (click to enlarge) to check how to specify grid size:

In case if current units setting in an active model differs from millimetres it is possible to specify grid dimensions using any other units, but actual dimensions should be close to mentioned above.
copy/paste the following function to a text field named "Z Function":
z_size*sin(0.02*(sqrt(((x-0.5*x_size)**2+(y-y_size*0.5)**2))))
Then click "put..." button to generate a grid. Grid points will follow entered function.

Note that in case if it is necessary to use a distance value within a function it is necessary to enter such value using decimal inches (SketchUp's internal distance unit). Or (and it is a more appropriate way) it is possible to use any of mentioned above variables (x_step, y_step, x_size etc) multiplied by some floating point factor/coefficient instead.