# [R] Mapping 2D to 3D

Abby Spurdle @purd|e@@ @end|ng |rom gm@||@com
Fri Sep 18 08:00:27 CEST 2020

```I'm not familiar with the gg graphics system.
However, I am familiar with density estimation, and density visualization.

There is *no* third variable, as such.
But rather, density estimates, which in this context, would usually be a matrix.
(And are computed inside the plotting or density estimation functions).

The documentation for the function you've used, says it uses  MASS::kde2d().
This does just that, returns an object, which contains a density matrix.
(Refer to the help file for kde2d).

Of course, there's no reason why one can't create a third variable,
from a mathematical perspective.
e.g. d, z, h, fv, or whatever you prefer...
And then set z = fh (x, y).

But there's no reason for the user to do that when using the plotting function.

Note that there are situations where one might want to set the limits
of the plot.
And set the breaks, colors, and color key.
e.g. Creating two or more plots, and putting them next to each other,
for comparison purposes.

On Fri, Sep 18, 2020 at 2:17 PM H <agents using meddatainc.com> wrote:
>
> I am trying to understand how to map 2D to 3D using ggplot() and eventually plot_gg(). I am, however, stuck on understanding how to express the third variable to be mapped. This example:
>
> ggdiamonds = ggplot(diamonds, aes(x, depth)) +
> stat_density_2d(aes(fill = stat(nlevel)),
> geom = "polygon", n = 100, bins = 10,contour = TRUE) +
> facet_wrap(clarity~.) +
> scale_fill_viridis_c(option = "A")
>
> uses a variable nlevel that I now understand is calculated during the building of the ggplot but I have not figured out from where it is calculated or how to specify a variable of my choosing.
>
> Does anyone have a good reference for understanding how to specify this variable? Most examples on the 'net seem to use the same dataset but do not specify this particular aspect...
>
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