There are two topics here:
1. Number of characters displayed in the "Output" window of a MATLAB Analysis or Visualization App page
2. Number of points to be visualized in a plot
The error message that is generated for your case is due to topic #1 - too many characters are being returned to the "Output" window.
Since I don't have your code, the general suggestion to resolve this error is to end your MATLAB commands with a semicolon ";". This prevents the LHS arguments from being printed to the "Output" window.
For example:
- Return characters to Output window
data = thingSpeakRead(12397)
- Do not return characters to Output window
data = thingSpeakRead(12397);
If this doesn't resolve the error, share your code and I might be able to find the cause of the error.
-Adarsh