Tracing and Diagnosing Layouts

Why trace a layout?

Most graph-layout functions return only the final coordinate matrix. That is enough for plotting, but it hides the most useful diagnostic information:

• how the multiscale hierarchy opened,
• whether the geometry stabilized gradually or abruptly,
• whether a parameter setting improved edge-length regularity or only moved the final picture around,
• and whether a weighted solve is actually getting closer to the intended metric geometry.

The trace.grip() API makes those questions visible through the same metric choice as grip():

• trace.grip(metric = "hop") for combinatorial layouts,
• trace.grip(metric = "edge_length") for edge-length geometry-aware layouts.

library(grip)
source(system.file('scripts','vignette-views.R',package='grip'),local=TRUE)

A first combinatorial trace

The example below traces a planar recursive triangle. Its nested holes make the multiscale construction visible. Using trace = "level" keeps the vignette readable by recording the coarse initialization, the start of each new level, and the final layout.

mesh.edges <- edges.sierpinski.triangle(4)

mesh.trace <- trace.grip(
  mesh.edges,
  n = max(mesh.edges),
  dim = 2,
  preset = "carpet",
  rounds = 48,
  final.rounds = 96,
  trace = "level",
  diagnostics = "light",
  seed = 1
)

knitr::kable(mesh.trace$meta)

knitr::kable(mesh.trace$diagnostics[, c(
  "frame",
  "edge.length.cv",
  "median.edge.length",
  "sampled.nonedge.sep.ratio"
)], digits = 3)

Each recorded stage has an ivue viewer showing its active vertices and edges. Drag to rotate or scroll to zoom. Original planar coordinates are retained at z = 0; each viewer fits its own bounds, so apparent size across stages is not a measure of expansion. When ivue is unavailable, a planar stage selector shows the same recorded vertices and edges with common bounds across stages.

The two most useful tables are:

• meta, which explains where each frame came from,
• diagnostics, which tracks simple quality signals frame by frame.

Level traces versus round traces

When you need more detail, trace = "round" records intermediate refinement rounds within each active level.

mesh.trace.round <- trace.grip(
  mesh.edges,
  n = max(mesh.edges),
  dim = 2,
  preset = "carpet",
  rounds = 6,
  final.rounds = 8,
  trace = "round",
  trace.every = 4,
  diagnostics = "light",
  seed = 1
)

data.frame(
  trace_mode = c("level", "round"),
  n.frames = c(length(mesh.trace$frames), length(mesh.trace.round$frames))
)
#>   trace_mode n.frames
#> 1      level        4
#> 2      round        8

Use:

• trace = "level" for most reporting and teaching,
• trace = "round" when you are diagnosing a tuning issue.

Read changes over the trace

Early frames contain only the active coarse vertices. Read changes in the score together with the stage and active graph; adjacent frames may not score identical vertex sets. Level traces show insertion stages; round traces show more of the refinement within each stage. They are different sampling schedules.

For a weighted graph, use trace.grip(metric='edge_length', ...) with its lengths. The weighted guide explains this metric. Detailed construction animations and their reproduction commands are described in the asset guide.

Trace objects are analysis objects

A trace object is more than a list of pictures. It can support:

• frame-by-frame diagnostics,
• manual inspection of where a solve destabilizes,
• teaching demonstrations of the multiscale hierarchy,
• and regression checks for future algorithm changes.

The most useful components are:

• final: the final coordinate matrix,
• frames: the stored coordinate frames,
• meta: frame provenance,
• diagnostics: per-frame metrics,
• lgkk.polish: optional post-layout LGKK result when enabled as an advanced experimental refinement step.

Practical guidance

• Start with trace = "level" and diagnostics = "light".
• Use trace = "round" only when you need finer-grained diagnostics.
• Supply target.coords when a meaningful geometric target exists.
• Prefer weighted traces for weighted graph families; they are much more interpretable than tracing the combinatorial solver on a graph with nontrivial edge lengths.

Where to go next

For an API map, see Finding your way around grip. For reproducible bundles and reference scoring, see Graph examples. These guides and the interactive-explorer walkthrough are installed vignettes.

• Getting Started with grip gives the short overview of the main APIs.
• Weighted Graph Layouts with grip focuses on weighted solving and scoring.
• Choosing Layouts for Real Data shows how trace fits into a real-data selection workflow.