The Serial Plotter needs numbers sent one line at a time, with the baud rate set to match the sketch. Text labels mixed with the numbers confuse it.
What the message means
The plotter reads each line as one sample. Values on one line separated by a comma, space or tab become separate traces.
Fix it in this order
- Set the baud rate in the plotter window to the value in Serial.begin().
- Send each sample with Serial.println() so a line ends after the numbers.
- For several traces print value1, a comma or space, then value2, then end the line.
- Close the Serial Monitor. Only one window can hold the port.
- Print only numbers and, if you want, labels in the form name:value.
- Print at a sensible rate. A flood of data makes the plot slow.
Why it happens
The Serial Plotter reads each line from the port, looks for numbers, and draws one trace per value. It only works when the sketch sends numeric text, one sample per line. Free text, such as a debug message, can stop the parser from matching the expected format. Each newline marks the end of a sample, so a print without a newline merges into the next sample.
Serial communication runs at a fixed speed, and both ends must agree on it. If the sketch calls Serial.begin with one rate and the plotter uses another, the bytes arrive as garbage and no valid numbers appear. Only one program can open a serial port at a time, so an open Serial Monitor can block the plotter. Matching the rate and closing other serial windows usually clears the problem.
If you see this, do this
| You see | Do this |
|---|---|
| Window stays empty | Baud mismatch or port in use. |
| Only one trace | Values are not separated on one line. |
Confirm the fix
- Upload a sketch that prints only a counter, such as Serial.println(i), and open the Serial Plotter; a rising line confirms the port works.
- Print two values on one line with a comma between them, like Serial.print(a); Serial.print(‘,’); Serial.println(b); two traces should appear.
- Confirm that Serial.begin’s baud value matches the rate selected in the plotter’s port setting, and that the Serial Monitor is closed.
Common mistake: Readers often assume the Serial Plotter shows any text, when it expects numbers in a regular, line-by-line format, so a sketch full of labels and messages looks empty.
Related
Serial Monitor shows nothing, Serial.print speed.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.