Grid Plot Layout#

Grid plot layout drives the --shape grid mode in blobtk, which renders a multi-panel scatter plot where each panel shows a windowed slice of a sequence. The feature was introduced in PR #12 "Feature/grid-plot-shape". The two primary source files are:

• rust/src/plot.rs — grid dimension calculation, GridSize struct, and the top-level plot_grid() orchestrator
• rust/src/plot/blob.rs — SVG rendering of the grid via blob::plot_grid()

Grid Dimension Calculation#

calculate_grid_size(num_items) returns (num_cols, num_rows) targeting the most square layout possible:

1. Start from floor(sqrt(num_items)) for both dimensions.
2. If items don't fit, increment num_rows (up to twice) to form an n×(n+1) or n×(n+2) rectangle.
3. If still not enough, increment num_cols and revert num_rows to produce an (n+1)×(n+1) square.

Cells are iterated column-major: col = i / num_rows, row = i % num_rows 1.

GridSize Struct#

GridSize is the central layout descriptor:

GridSize::new() computes these values with the following hard-coded defaults 2:

• Inner margin (bottom/left): 25 px
• Padding: 10 px on all sides
• Outer margin (bottom/left): 50 px

row_height is (height − outer_bottom − outer_top) / num_rows 3.

Column Width Calculation#

Column widths support two modes 4:

• Uniform (no ratios): col_width = (width − outer_margins) / num_cols. Each column gets the same width.
• Proportional (ratios provided): Each column width is scaled by ratio[i] / sum(ratios) × num_cols, then padded with margin/padding. This is used when x_field = "position" and no explicit x_limit is set, so columns reflect actual genomic span 5.

When x_field = "position", ratios are derived from the per-column maximum x-value relative to the global x-max, rounded to the nearest tenth 6. Axis x-limits are then scaled per column by limits["x"][1] * ratio[col] so each panel's x-axis reflects only the span of its column's sequences 7.

Sequence Title Truncation#

Each subplot optionally displays a sequence title sourced from BlobData.title 8. Truncation logic in blob::plot_grid():

• Condition: title.len() > 3 AND title.len() * 10.0 > col_widths[col]
• Truncation: title[..(col_width / 10.0) as usize − 3] + "..."
• Heuristic: ~10 pixels per character (no actual font measurement)

Titles are rendered as SVG <text> elements at 75% of the base font size (20 px → 15 px), centered horizontally at the top of each cell using text-anchor: middle and dominant-baseline: hanging 9.

Axis Label Suppression#

To reduce clutter, tick labels are suppressed selectively 10:

• X-axis tick labels: shown only on the last row, or the final item.
• Y-axis tick labels: shown only on the first column (col == 0).

Global x/y axis labels are rendered once, outside all cells, centered against the full grid dimensions 11.