reglscatterpy 0.6.2

Interactive WebGL scatterplots for single-cell data (AnnData/MuData/SpatialData) in Jupyter, VS Code and Shiny for Python

reglscatterpy

Interactive WebGL scatterplots for single-cell / spatial data in Python — AnnData, MuData, SpatialData, pandas, numpy. Renders millions of points in the browser via regl-scatterplot, in Jupyter, JupyterLab, VS Code and Colab.

This is the Python companion to the R package reglScatterplotR. Both drive the same compiled widget, so a plot looks and behaves identically across R and Python — the draggable legend, filter_by distribution sliders, lasso, tooltips and PNG/SVG/PDF export all come from one shared codebase. (Equivalence is locked down by tests/test_payload_parity.py, which checks the Python payload byte-for-byte against R fixtures.)

Install

pip install reglscatterpy            # numpy, pandas, anywidget
pip install anndata                  # for AnnData; mudata / spatialdata as needed

Quick start

import scanpy as sc
import reglscatterpy as rs

adata = sc.datasets.pbmc3k_processed()
rs.scatterplot(adata, basis="umap", color_by="louvain")   # an obs column
rs.scatterplot(adata, basis="umap", color_by="CST3")      # a gene

basis= selects the embedding for single-cell objects — short names like "umap"/"pca" resolve to the obsm key (X_umap, …), case-insensitively. x="X_umap" still works as an alias. For a DataFrame you instead give the coordinate columns with x=/y=:

import numpy as np, pandas as pd
df = pd.DataFrame({"x": np.random.rand(10_000), "y": np.random.rand(10_000),
                   "ct": np.random.choice(list("ABC"), 10_000)})
rs.scatterplot(df, x="x", y="y", color_by="ct")

Plots are 700 px wide by default (not the full cell width). Pass width= (pixels) for a different size, or width=None to fill the cell.

Big data: atlas-scale rendering

By default scatterplot() keeps huge datasets interactive without silently hiding cells, controlled by max_points (default "auto"):

# AUTO (default): caps at 500k via a density-preserving subsample.
rs.scatterplot(adata, basis="umap", color_by="cell_type")
# -> on-plot caption "500,000 of 3,900,000 shown" + a one-time warning.

The "auto" subsample uses a 2-D grid density sketch (subsample="density") that thins dense blobs but keeps rare cell types — unlike uniform random sampling, which drops them (subsample="random" is the uniform fallback). The plot is always honest about it: the "X of Y shown" caption is drawn on the figure, repr() reflects it, and an automatic downsample warns once. w.selection still indexes the original rows.

# ALL POINTS RESIDENT (the Allen ABC-Atlas method): every cell on the GPU,
# camera-only pan/zoom. Smooth up to ~4M cells on a decent GPU.
rs.scatterplot(adata, basis="umap", color_by="cell_type", max_points=None)

rs.scatterplot(adata, basis="umap", color_by="cell_type", max_points=1_000_000)

For datasets larger than ~4M (where all-resident gets heavy), use progressive=True — detail-on-zoom, an in-memory tiling with no preprocessing:

rs.scatterplot(adata, basis="umap", color_by="cell_type", progressive=True)

It shows a light density-sketch overview, then re-renders all cells inside the viewport as you zoom in (a zoomed view holds few cells, so they draw at full detail with a complete lasso). The camera domain stays fixed and the overview snaps back instantly on zoom-out. Tune it with progressive_opts:

rs.scatterplot(adata, basis="umap", color_by="cell_type", progressive=True,
               progressive_opts={"detail_max_points": 300_000, "overscan": 0.6})

• detail_max_points — max points per zoomed-in viewport (lower = smoother pan; defaults to max_points/500k).
• overscan — fraction of margin fetched around the view, so panning has no hard cuts (lower = lighter pan, more visible edges; default 0.6).

Rule of thumb: max_points=None for ~2–4M real atlases; progressive=True only beyond that. progressive=True always uses the live (interactive) widget.

Gallery

Categorical colouring	Continuous (gene) colouring
filter_by distribution sliders	Linked grid (compose)

Static by default, interactive on request

By default a plot renders as a self-contained snapshot (a sandboxed <iframe> with the WebGL bundle and data baked in) — like a plotly figure, it shows in JupyterLab, Notebook 7, VS Code and Colab, and survives reopening the notebook with no kernel (no re-run, no blank widget-view). It stays fully interactive visually — pan, zoom, lasso, legend, tooltips, PNG/SVG/PDF export — but, having no kernel link, it can't send a selection back to Python.

For the Python round-trip (w.selection, annotate, diff_expression, linked compose grids) pass interactive=True to get the live, kernel-linked widget:

w = rs.scatterplot(adata, basis="umap", color_by="leiden", interactive=True)
w                          # lasso some cells…
adata[w.selection]         # …read them back in Python

The live widget needs a running kernel (and, like any Jupyter widget, may show blank on reopen). The static default does not — so use the default for figures you want to keep/share, and interactive=True while you're actively selecting.

Save a standalone HTML (offline, kernel-free)

The Python equivalent of R's htmlwidgets::saveWidget: write a single self-contained .html that inlines the widget and the plot's data, so it opens in any browser with no kernel and no internet:

w = rs.scatterplot(adata, x="X_umap", color_by="leiden")
rs.save_html(w, "umap.html")      # or:  w.to_html("umap.html")

The saved file is fully interactive (pan/zoom, legend, lasso, tooltips, PNG/SVG/PDF export) but it's a snapshot — it has no kernel, so the Python round-trips (w.selection, w.annotate, …) only work in the live notebook. The widget bundle is inlined gzip-compressed (~0.5 MB, decompressed in-browser), so a one-plot file is well under 1 MB. No R is involved — it's pure Python.

A whole notebook → one HTML report (no re-running)

Plain jupyter nbconvert --to html leaves the plots blank (the same widget-state limitation). The fix that avoids re-executing a heavy notebook is record mode: call rs.record_html() once at the top, then run your notebook normally — each plot bakes a static, interactive copy into its own cell output. After that:

import reglscatterpy as rs
rs.record_html()                 # run once near the top, then work as usual
# ... rs.scatterplot(...) cells ...

# reopening the notebook now shows the plots, and either of these makes a report
# WITHOUT re-running anything:
jupyter nbconvert --to html analysis.ipynb
reglscatterpy-report analysis.ipynb -o analysis_report.html

reglscatterpy-report (and rs.save_notebook_html(...)) default to not re-executing — they use the recorded outputs and share one copy of the bundle across all plots. For a notebook that wasn't recorded, pass --execute (CLI) / execute=True to re-run it once.

rs.save_notebook_html("analysis.ipynb", "report.html")             # uses outputs
rs.save_notebook_html("analysis.ipynb", "report.html", execute=True)  # re-runs

Recorded plots are a one-way snapshot: pan/zoom/lasso/tooltips/export all work, but w.selection / w.annotate no longer round-trip to Python (there's no kernel). Call rs.record_html(False) to go back to the live widget.

Needs nbconvert + ipykernel (pip install 'reglscatterpy[report]'). The plots are fully offline; nbconvert's own page chrome (MathJax/RequireJS) is still CDN-referenced — use nb_offline_convert if you need the surrounding report shell to be 100% offline too.

Selection round-trip

Lasso points in the plot, then read them back in another cell — or drive the selection from Python. This needs the live widget, so pass interactive=True:

w = rs.scatterplot(adata, basis="umap", color_by="leiden", interactive=True)
w                          # show it, lasso some cells in the widget

w.selection                # -> [12, 87, 134, ...]  positional indices
adata[w.selection]         # subset the AnnData directly
sub = w.subset()           # same thing, as a convenience

w.selection = list(range(100))   # or set it from Python to highlight points

Annotate cells by lassoing

Lasso a population, label it, and the label is written straight back into adata.obs (or a DataFrame column) — curate cell types interactively:

w = rs.scatterplot(adata, basis="umap", color_by="leiden", interactive=True)
w                                  # lasso a cluster
w.annotate("cell_type", "T cells") # -> writes adata.obs["cell_type"] for those cells
# lasso another, w.annotate("cell_type", "B cells"), ... then:
rs.scatterplot(adata, x="X_umap", color_by="cell_type")

Differential expression of a selection

Lasso a population and get its top markers vs the rest (or vs another lasso):

w = rs.scatterplot(adata, basis="umap", color_by="leiden", interactive=True)
w                          # lasso a cluster
w.diff_expression(n=10)    # top genes for the selection vs all other cells
# or two saved selections:
a = w.selection            # after lassoing group A
# (lasso group B)
w.diff_expression(a, w.selection)

Richer tooltips

Show extra fields on hover:

rs.scatterplot(adata, x="X_umap", color_by="leiden",
               tooltip_by=["n_genes", "sample", "CST3"])   # obs cols or genes

Composition of a selection

Lasso a region and see what it's made of:

w = rs.scatterplot(adata, basis="umap", color_by="leiden", interactive=True)
w                                  # lasso a region
w.composition("leiden")            # -> count + fraction per cluster in the selection

Outlines & highlighting

Two ways to make points stand out, both using a crisp antialiased ring:

add_outline=True — a scanpy-style outline on every point (whole-plot aesthetic). The body keeps its colour; an outer ring + background gap go behind it:

rs.scatterplot(adata, basis="umap", color_by="cluster", add_outline=True)
rs.scatterplot(adata, basis="umap", color_by="cluster", add_outline=True,
               outline_color=("black", "white"), outline_width=(0.3, 0.05))

add_outline is for small/medium plots — it auto-skips (with a warning) above ~150k drawn points, where the ring is invisible (~1px) anyway. For huge data, mark a subset with highlight instead.

w.highlight([...]) — persistently mark a chosen subset of cells (live widget). Unlike a lasso selection, it survives double-click and new lassoes:

w = rs.scatterplot(adata, basis="umap", color_by="cluster", interactive=True)
w.highlight([12, 87, 134], color="red")   # ring + size bump on these cells
w.highlight([])                            # clear

Indices are original data indices (e.g. np.where(adata.obs["cluster"]=="T")[0]).

Linked grid

Colour one embedding by several genes / obs columns at once — pass a list to color_by and you get a linked grid, one panel per value (camera + lasso stay in sync across panels):

rs.scatterplot(adata, basis="umap", color_by=["louvain", "CST3", "NKG7"])

A list of names means "one panel per name". A raw per-point colour vector must be a numpy array / pandas Series (not a list of strings).

Or compose pre-built plots — e.g. compare different embeddings side by side. compose() auto-upgrades plain plots to live widgets, so you don't need interactive=True on each:

from reglscatterpy import scatterplot, compose

a = scatterplot(adata, basis="umap", color_by="leiden")
b = scatterplot(adata, basis="pca",  color_by="leiden")
compose([a, b])            # 2-up grid, linked camera + selection

A lasso or legend-category filter on one panel propagates to the others by original cell — even across panels coloured by different variables, and even when the panels are progressive=True (each panel fetches detail for the synced viewport). A view reset on one panel resets the whole group.

Toolbar & selection extras

scatterplot(..., toolbar="left") (or "top", "none") shows an in-plot toolbar: pan, lasso, zoom-to-selection, reset, screenshot. Pass zoom_on_selection=True to auto-frame a lasso selection.

Encode a numeric column or a gene on point size or opacity (in addition to colour): scatterplot(adata, basis="umap", color_by="leiden", size_by="n_genes"), size_by="CST3", or opacity_by="total_counts".

Supported objects

Input	x (embedding)	color_by / group_by
AnnData	obsm key ("X_umap", "umap", "spatial", …)	obs column or var_names feature
MuData	global obsm or "modality:embedding"	obs column or "modality:feature"
SpatialData	table's obsm (defaults to "spatial")	table's obs / features
pandas.DataFrame	column name	column name or vector
numpy.ndarray	column index	vector

API parity with R

rs.scatterplot(...) mirrors R's reglScatterplot(...): color_by / group_by, point_size, opacity, point_color, pixel_ratio, continuous_palette / categorical_palette, custom_colors, vmin / vmax, center_zero, filter_by, legend styling, enable_download, and more.

A backend="jscatter" option also exists if you'd rather render with jupyter-scatter (pip install reglscatterpy[render]); the default native widget is recommended.

The widget bundle

src/reglscatterpy/static/widget.js is a built artifact (an anywidget ESM bundle). Its source — the shared rendering widget plus the anywidget adapter — lives in the reglScatterplotR repo under js/. To refresh it after a JS change, build there and copy the result here:

# from a sibling checkout of reglScatterplotR
cd reglScatterplotR/js && npm install && npm run build
cp dist/widget.js ../../reglscatterpy/src/reglscatterpy/static/widget.js

Develop / test

pip install -e .[dev]
pytest          # extraction tests skip cleanly without anndata/scipy