How to Extract Data from a Graph or Chart Image

You have a chart — in a scanned PDF, a textbook, an old report, a screenshot — but not the numbers behind it. Here’s how to get them back in a few minutes, free, without installing anything, using the Sutranet Graph Digitizer.

What does “digitizing a graph” mean?

Digitizing a graph (also called plot digitizing or data extraction) means recovering the underlying numbers from a picture of a plot. You tell the tool where the axes are and what they read, click along the curve, and it converts those pixels into real (x, y) values you can use in a spreadsheet or calculation.

Step 1 — Load the graph image

Open the Graph Digitizer and get your chart onto the canvas: drag an image file in, paste a screenshot with Ctrl‑V, or use Load image. Any common image format works. Nothing is uploaded — the file stays on your device.

Step 2 — Calibrate the axes

The tool needs to know what the axes read. Click Calibrate and it walks you through four points: two on the X axis and two on the Y axis. For each, click a spot where you know the value (usually a gridline), then type that value.

Set each axis to linear or log first. That single choice is what lets the tool handle semi‑log and log‑log charts correctly — a value halfway up a log decade is read as the geometric mean, not the arithmetic one.

Step 3 — Trace the curve

Switch to Trace mode and click along the curve. You don’t have to be pixel‑perfect: each click snaps onto the centre of the nearest line, so a rough click still lands on the curve. A magnifier follows the cursor for precise work. Tracing a family of curves? Add a separate named curve for each and they export as separate columns.

Step 4 — Read a value, or export to Excel

Now you have two ways to get what you need:

• Read one value: type an x into Read a value and get y straight off the fitted curve — no more squinting at gridlines. A marker shows the read‑off point on the chart.
• Get the whole dataset: Copy table pastes the numbers straight into Excel or Google Sheets, or Download .csv saves them as a file.

Step 5 — Save and come back later

Use Save session to keep your work — it stores the image, calibration and every traced point in one file. Reopen restores the whole session so you can pick up exactly where you left off.

Tips for accurate results

• Use the clearest, highest‑resolution image you can — sharper lines digitize more accurately.
• Trace more points where the curve bends sharply, fewer where it’s straight.
• Turn on Overlay fitted curve and check the drawn curve sits right on top of the printed line before trusting the numbers.
• For a smooth underlying curve, a low‑degree polynomial fit averages out clicking jitter; otherwise keep the faithful piecewise‑linear default.

Is my data private?

Yes. The Graph Digitizer runs entirely in your browser. Your image and the data you extract are processed on your own device and are never uploaded to us or anyone else. (We record only an anonymous count of how many people use the tool — no content, no personal data. See our privacy policy.)

Frequently asked questions