Slack Block Kit Charts: Line, Bar, Area and Pie, Tested

To send a chart in a Slack message, post a data_visualization block with chat.postMessage or an incoming webhook. It draws a line, bar, area or pie chart from numbers in your JSON, so you no longer render a PNG and upload it. Slack added the block in June 2026, a month after the sortable data_table block. On 30 September 2026 we posted 29 chart and table payloads to a one-person test workspace and recorded what rendered and what Slack refused.

The smallest chart that works

This is the script we ran with a bot token that has chat:write. The same chart dict went to an incoming webhook in the second call:

import os
import requests
from slack_sdk import WebClient

client = WebClient(token=os.environ["SLACK_BOT_TOKEN"])
days = ["Mon", "Tue", "Wed", "Thu", "Fri"]
signups = [12, 18, 9, 22, 17]
chart = {
    "type": "data_visualization",
    "title": "Signups this week",
    "chart": {
        "type": "bar",
        "series": [{"name": "Signups", "data": [{"label": d, "value": v} for d, v in zip(days, signups)]}],
        "axis_config": {"categories": days, "y_label": "Signups"},
    },
}
resp = client.chat_postMessage(channel=os.environ["CHANNEL"], text="Signups this week: 78", blocks=[chart])
print("bot token:", resp["ok"], resp["ts"])

r = requests.post(os.environ["SLACK_WEBHOOK_URL"], json={"text": "Signups this week: 78", "blocks": [chart]}, timeout=30)
print("webhook:", r.status_code, r.text)

Output:

bot token: True 1790771328.614459
webhook: 200 ok

Both calls drew the chart, and so did chat.postMessage with a user token. Block Kit Builder also previews the block; it now has a Data Visualization entry in its left menu:

Three parts are required: title, chart.type, and, for line, bar and area, axis_config.categories. Each series needs one data point for every category, in any order; the categories array sets the order on the x axis.

The top-level text is what notifications and screen readers get. When we left it out, the post still succeeded and Slack stored the chart title ("Open incidents") as the message text, while slack_sdk printed a warning.

Bar, area and pie charts as Slack draws them

Bar charts group bars by category, one colour per series, and negative values go below the zero line. The Infra bar below is -5:

Area charts fill under each line. With 20 categories, the most Slack accepts, the x labels turn diagonal:

Pie charts take segments instead of series and axis_config. Slack works out the percentages from the values:

{"type": "data_visualization", "title": "Plan mix",
 "chart": {"type": "pie", "segments": [
   {"label": "Free", "value": 61}, {"label": "Pro", "value": 30}, {"label": "Business+", "value": 9}]}}

Slack's docs say a segment value must be greater than 0. A segment with value 0 was accepted anyway: it showed in the legend and drew no slice. Every line in all four chart types is smoothed into a curve, and we found no option to draw straight segments.

Limits Slack enforced, with the errors

Every rejected payload came back as invalid_blocks, with the reason in errors. These are the exact strings we got:

The generic schema error names only /blocks/0/chart, not the field. If you see it, check these counts first: at most 12 series, 20 categories, 12 pie segments, and 20 characters per label or series name.

Charts update in place. We posted a line chart, then called chat.update with new values on the same ts, and the message redrew with the new last point. That makes one message per day a workable live dashboard.

The data_table block for rows of numbers

For the table part of a report, the data_table block gives a sortable, paged table. It is a different block from the older table block covered in how to make a table in Slack. Our 8-row table showed 5 rows and a pager, because page_size defaults to 5; hovering showed search, expand and menu buttons:

{"type": "data_table", "caption": "Service health, last 24 h",
 "rows": [
   [{"type": "raw_text", "text": "Service"}, {"type": "raw_text", "text": "p95 ms"}],
   [{"type": "raw_text", "text": "api"}, {"type": "raw_number", "value": 120, "text": "120"}]
 ]}

What the API refused:

Header plus 200 rows went through. Columns made only of raw_number cells sort as numbers; mixed columns sort as text.

FAQ

Can I preview a chart before sending it?

Yes. Block Kit Builder draws it, as in the screenshot above. Post to a DM with yourself as well, because the error strings in the tables only come back from the real API.

What does a bad chart look like from a webhook?

The webhook answered our broken chart (no axis_config) with HTTP 400 and the plain-text body invalid_blocks, without the errors detail. Post the same blocks with chat.postMessage to see which field is wrong. How to read webhook errors is in the Slack webhook curl example.

Do I need a special scope?

No. chat:write for a bot token, or the incoming-webhook scope for a webhook URL, was enough. The bot token guide covers getting one.

Can a chart have clickable points or links?

The block's documented fields are only type, title, chart and block_id, so there is no place for actions or links. Put a button or link in a separate section or actions block under the chart.