Plot pair-wise distances between individuals as line segments.

## Arguments

- D
distance matrix, should be of class 'dist' or compatible class

- idx
an integer sequence defining which individuals should be compared

## Value

No value is returned.

## Details

By default all individuals are plotting on the same axis. When there are
more than about 10 individuals, the plot can become quite messy. See
examples below for ideas.

## References

https://casoilresource.lawr.ucdavis.edu/

## Examples

```
data(sp2)
depths(sp2) <- id ~ top + bottom
d <- NCSP(
sp2,
vars = c('prop', 'field_ph', 'hue', 'value'),
maxDepth = 100,
k = 0.01
)
#> Computing dissimilarity matrices from 18 profiles
#> [0.4 Mb]
par(mfcol=c(3,1), mar=c(2.5,4.5,1,1))
plot_distance_graph(d, idx=1:6)
plot_distance_graph(d, idx=7:12)
plot_distance_graph(d, idx=12:18)
```