better-rtplot 0.2.5

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rtplot — real-time plotting over ZMQ

rtplot lets a Python script push live data to a plot window — locally, or across the network — with a few lines of code on the sender side. The plot window runs in any modern browser and supports interactive controls (buttons, sliders, dials, text and numeric displays) that feed values back into the sending script in real time.

Typical use: a robot or data-acquisition script runs on a Raspberry Pi or microcontroller host, and you watch live signals and tweak gains from a laptop on the same network.

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Table of contents

• Highlights
• Install
• 60-second quickstart
• Interactive controls
  • Reading controls from Python
  • Pushing values into displays
  • Element reference
• Plot configuration
• Sending data
• Saving data
• Networking modes
• Viewing the plot from another device
• Performance tuning
• CLI reference
• Examples

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Highlights

• Fast. Binary WebSocket deltas push data at up to 1 kHz. The browser coalesces incoming samples into a single repaint per requestAnimationFrame, so rendering runs at your monitor's refresh rate (typically 60 Hz, 120 Hz on higher-refresh displays) regardless of how fast samples arrive.
• Browser-based. The plot window is served by aiohttp and rendered by uPlot in any modern browser. No desktop GUI toolkit to install, works over SSH port forwarding out of the box.
• Remote-friendly. Either the sender or the plot host can bind — pick whichever fits your network. Works across LAN, WSL, and SSH tunnels.
• Plot config lives with the data. The sender declares the plot layout, so a Pi running your experiment owns the look of its own dashboards.
• Interactive controls. Declare buttons, sliders, dials, numeric/text displays in the same initialize_plots call. Poll from your tight loop; no threads, no callbacks.
• Save to Parquet with a single button click or client.save_plot() call.

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Install

Install rtplot with the server bundle — this is the normal path and gets you everything:

pip install "better-rtplot[browser]"

This pulls aiohttp (for serving the plot UI) plus pandas + pyarrow (for saving runs to Parquet). If you only need the sender side — your script pushes data to someone else's plot host and you don't run a server locally — you can install the client-only minimum instead:

pip install better-rtplot

In that case, if you later try to launch a server locally you'll get a clear error telling you to add the [browser] extra.

WSL users: nothing extra needed. The plot window is served by HTTP, so just open the URL rtplot prints in your Windows browser.

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60-second quickstart

Terminal 1 — start the plot server:

python -m rtplot.server_browser

It prints a URL like http://localhost:8050 — open that in your browser. The page stays blank until a client sends a plot config.

Terminal 2 — send data:

from rtplot import client
import numpy as np, time

client.local_plot()                     # send to the server on 127.0.0.1
client.initialize_plots(["sin", "cos"]) # one plot with two named traces

for i in range(10000):
    t = i * 0.01
    client.send_array([np.sin(t), np.cos(t)])
    time.sleep(0.01)

That's it. The browser tab you opened will start drawing the two traces in real time.

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Interactive controls

Declare a control row inline in your plot layout:

from rtplot import client
import numpy as np, time

client.local_plot()
client.initialize_plots([
    {"names": ["signal"], "yrange": [-6, 6]},
    {"controls": [
        {"type": "button", "id": "reset", "label": "Reset"},
        {"type": "button", "id": "pause", "label": "Pause"},
        {"type": "slider", "id": "gain",  "label": "Gain",
         "min": 0, "max": 5, "value": 1.0, "step": 0.1, "format": "{:.2f}"},
    ]},
    {"controls": [
        {"type": "dial",    "id": "freq", "label": "Freq (Hz)",
         "min": 0.1, "max": 5.0, "value": 1.0, "step": 0.05,
         "sensitivity": 0.5, "format": "{:.2f}"},
        {"type": "display", "id": "t",    "label": "t (s)", "format": "{:.2f}"},
        {"type": "text",    "id": "msg",  "label": "Status",
         "value": "running"},
    ]},
])

running = True
t0 = time.time()
while True:
    ctrl = client.poll_controls()
    for btn in ctrl.buttons:
        if btn == "reset": t0 = time.time()
        if btn == "pause": running = not running

    gain = ctrl.values.get("gain", 1.0)
    freq = ctrl.values.get("freq", 1.0)
    t = time.time() - t0
    amp = gain * np.sin(2 * np.pi * freq * t) if running else 0.0

    client.set_display("t", t)
    client.set_display("msg", "paused" if not running else "running")
    client.send_array(amp)
    time.sleep(0.01)

Reading controls from Python

ctrl = client.poll_controls()           # non-blocking, cheap to call every loop
gain = ctrl.values.get("gain", 1.0)     # latest slider/dial value
for btn_id in ctrl.buttons:             # list of buttons fired since last poll
    handle(btn_id)

poll_controls() returns a ControlState(values, buttons) namedtuple:

• values — a dict of {element_id: float} for every slider and dial the server has told the client about. Defaults declared in initialize_plots are pre-seeded so the first call already sees them.
• buttons — a list of button ids fired since the previous poll, in order. The list is cleared on return, so each event is delivered exactly once.

Call it from your tight loop before computing the next sample. No threads, no callbacks, no missed events.

Pushing values into displays

client.set_display("t", 12.34)       # numeric display box
client.set_display("msg", "running") # text field

set_display() accepts either a number (for type: "display" elements) or a string (for type: "text" elements). Updates are coalesced on the server and rebroadcast to every connected browser at ~30 Hz.

Element reference

Type	Purpose	Notable fields
button	Fires a discrete event when clicked	id, label
slider	Scalar input via horizontal range	id, label, min, max, value, step, format
dial	Scalar input via rotational drag	same as slider, plus sensitivity (full turns per range sweep; default 1.0)
display	Read-only numeric readout	id, label, format
text	Read-only text field (prompts, status)	id, label, value

Slider and dial widgets both render as [widget] [−] [number input] [+], so you can drag, type a value directly, or nudge by step. The dial accepts "round and round" circular drag — each full rotation walks the value through (max − min) × sensitivity, so sensitivity: 0.25 gives you four rotations per sweep for fine control.

The format field accepts Python-style {:.Nf} strings (e.g. "{:.2f}").

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Plot configuration

Each entry in initialize_plots is one of:

• an integer — client.initialize_plots(3) → one plot with 3 anonymous traces
• a string — client.initialize_plots("torque") → one plot with one named trace
• a list of strings — one plot, one trace per name
• a list of lists of strings — one plot per sublist
• a dict — one plot, with full styling options (below)
• a list of dicts — multiple plots with full styling

A styled plot dict accepts any of:

Key	Meaning
names	Required. List of trace names.
colors	List of per-trace colors. Single letter (r g b c m y k w) or any CSS color string.
line_style	"-" for dashed, "" (or anything else) for solid, per trace.
line_width	Per-trace line width in pixels.
title	Plot title.
xlabel / ylabel	Axis labels.
yrange	[ymin, ymax] — pins the Y axis and significantly speeds up rendering.
xrange	Integer number of samples visible at once (default 200).

Special row entries (not plots themselves):

• {"controls": [...]} — a row of interactive controls (see Interactive controls)
• {"non_plot_labels": ["name1", "name2"]} — extra scalar names that ride along with send_array and get saved into the output Parquet file, but aren't rendered as traces

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Sending data

client.send_array(scalar)           # float
client.send_array([a, b, c])        # 1-D list: one sample per trace
client.send_array(np.array([...]))  # 1-D numpy array: one sample per trace
client.send_array(np.array([[...]]))# 2-D (num_traces, N): N samples at once

Passing a 2-D array with N > 1 lets you push a batch of samples per send_array call, which is the fastest way to get many samples through without dropping frames.

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Saving data

The server saves every sample it has received since the latest initialize_plots call to a Parquet file, including any non_plot_labels data that rode along with your normal data.

Trigger a save from either side:

• Browser UI: click the Save Plot button.
• Python: client.save_plot("my_run")

Control where things get written:

python -m rtplot.server_browser -sd ./saved_plots -sn experiment1

• -sd / --save-dir — target directory
• -sn / --save-name — filename prefix (a timestamp is always appended)

Save non-plot signals alongside the plotted ones

client.initialize_plots([
    {"names": ["hip_angle", "knee_angle"]},
    {"non_plot_labels": ["battery", "cpu_temp", "loop_latency"]},
])

Send battery, cpu_temp and loop_latency as extra rows after the plotted traces in each send_array call; they won't be drawn but they will land in the Parquet file.

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Networking modes

rtplot uses ZMQ, so either the sender or the plot host can be the one that binds a socket. Pick whichever works for your network and firewalls.

Mode A — plot host binds, sender connects (typical for lab laptops)

# on the plot host (e.g. your laptop)
python -m rtplot.server_browser

# on the sender (e.g. the Pi)
from rtplot import client
client.configure_ip("192.168.1.42")   # the laptop's LAN IP

Mode B — sender binds, plot host connects (typical when the sender has a static IP and the viewer roams around)

# on the plot host
python -m rtplot.server_browser -p 192.168.1.50   # the sender's IP

# on the sender
from rtplot import client
# no configure_ip call needed — the default behavior binds

If you pass -p host:port to the server, rtplot also derives the control return-channel endpoint from that same host/port (it uses port+1). This means sliders, buttons, and dials work transparently in both modes with no extra config.

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Viewing the plot from another device

The section above is about the link between your sender script and the plot host (the machine running rtplot.server_browser). This section is about the other relationship: the link between the plot host and a separate viewer device — a phone, tablet, or another laptop that just wants to open the browser UI.

You don't need SSH for this. The plot host already runs a plain HTTP server on port 8050, bound to every interface, and the viewer device is only a web browser. All you need to do is get traffic from the viewer to port 8050 on the plot host.

On the same LAN (phone, tablet, another laptop on the same Wi-Fi)

1. Find the plot host's LAN IP:

   ipconfig | findstr IPv4       # Windows
   

   ip -4 addr | grep inet        # Linux/WSL
   

2. Open http://<lan_ip>:8050 in the browser on the viewer device.

3. If Windows, allow inbound connections on port 8050 through Windows Defender Firewall. The very first time you run python -m rtplot.server_browser, Windows pops up an "Allow Python to receive connections" dialog — tick Private networks and click Allow. If you missed the dialog, add the rule manually from an elevated PowerShell:

   # PowerShell as Administrator
   New-NetFirewallRule -DisplayName "rtplot" `
       -Direction Inbound -LocalPort 8050 -Protocol TCP `
       -Action Allow -Profile Private
   

   Only allow on Private (home / trusted Wi-Fi), not Public, unless you know what you're doing. To remove the rule later:

   Remove-NetFirewallRule -DisplayName "rtplot"
   

No router configuration, no SSH tunneling, no external accounts. Just a firewall exception.

WSL2 wrinkle

If you run the server inside WSL2 instead of native Windows, WSL2's localhost auto-forward lets you reach it from your Windows browser, but does not forward traffic from the LAN. To expose a WSL2-hosted server to other devices you need one extra hop — a Windows-side port proxy that forwards incoming LAN traffic into WSL2:

# PowerShell as Administrator
$wslIp = (wsl hostname -I).Trim().Split()[0]
netsh interface portproxy add v4tov4 `
    listenport=8050 listenaddress=0.0.0.0 `
    connectport=8050 connectaddress=$wslIp
New-NetFirewallRule -DisplayName "rtplot wsl" `
    -Direction Inbound -LocalPort 8050 -Protocol TCP `
    -Action Allow -Profile Private

WSL2's IP changes on every reboot, so rerun the netsh line after a restart (or just run rtplot.server_browser from native Windows and skip this whole step).

To undo:

netsh interface portproxy delete v4tov4 listenport=8050 listenaddress=0.0.0.0
Remove-NetFirewallRule -DisplayName "rtplot wsl"

Across the internet (viewer on cellular, another network, etc.)

Two easy options, neither of which requires touching your router:

Cloudflare Tunnel (free, one-shot URL):

winget install --id Cloudflare.cloudflared
cloudflared tunnel --url http://localhost:8050

Prints an https://<random>.trycloudflare.com URL valid for the lifetime of the command — paste it into the viewer's browser. Kill the command when you're done.

Tailscale (private mesh VPN, best for recurring setups):

Install Tailscale on both the plot host and every viewer device. Each device gets a stable 100.x.y.z IP that works from any network. Open http://100.x.y.z:8050 on the viewer.

Both tunnel paths forward the HTTP + WebSocket traffic that the browser needs; neither involves ZMQ, since the viewer is browser-only. Your sender script keeps talking to the plot host locally as usual.

Ports at a glance

Port	What it's for	Who actually needs it open
8050 (TCP)	HTTP + WebSocket to the browser UI	the plot host, inbound from viewers
5555 (TCP)	ZMQ data (sender → server)	only the sender and the plot host
5556 (TCP)	ZMQ control return channel (server → sender)	only the sender and the plot host

For the "other device is a viewer" case, you only need to expose 8050. 5555 / 5556 are between the sender script and the plot host — they do not need to be reachable from the viewer device at all.

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Performance tuning

If you start running out of frames, try these, in roughly this order:

1. Pin the Y range. {"yrange": [-2, 2]} on each plot lets the renderer skip autoscaling work and gives the single biggest win.
2. Batch your samples. Pass a 2-D numpy array to send_array so N samples ship per call.
3. Shrink the window. Fewer pixels to redraw per frame.
4. Reduce line_width. Thicker lines cost more to rasterize.
5. Use the -s N / --skip N server flag to push every Nth sample batch to the browser instead of every one. Add -a / --adaptable to let the server tune N to your data rate automatically.
6. Increase xrange. Counterintuitively, a longer visible history can be cheaper than a short one because the browser ring-buffers the data and only replaces the tail on each push.

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CLI reference

python -m rtplot.server_browser accepts:

Flag	Default	Meaning
-p HOST[:PORT]	(bind)	Connect to a sender at this address instead of binding
--host HOST	0.0.0.0	HTTP bind interface
--port N	8050	HTTP port
--no-browser	off	Don't try to open a browser on startup
--rate N	1000	Max WebSocket push rate (Hz)
-n N / --skip N	1	Push every Nth sample batch
-a / --adaptable	off	Auto-tune skip rate to data rate
-c / --column	row	Lay plots out in columns instead of rows
-d / --debug	off	Extra debug logging
-sd DIR / --save-dir DIR	cwd	Where to write .parquet saves
-sn NAME / --save-name NAME	—	Prefix for saved filenames

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Examples

• rtplot/example_code.py — a walk through every initialize_plots signature, plus a controls demo at the bottom.

• rtplot/interactive_test.py — a guided end-to-end test that walks you through clicking buttons, dragging sliders, typing into the number input, using the ± nudge arrows, and spinning the dial. Good for smoke-testing a fresh install.

  python -m rtplot.server_browser &
  python -m rtplot.interactive_test