Communicate to user when Serial Plotter limitations exceeded

[per1234]

Describe the current behavior

The Arduino IDE 2.x Serial Plotter has an undocumented maximum limit of 8 variables.

When this limit is exceeded, the plotter is a blank window, with no indication to the user of the cause of this behavior.

To Reproduce

1. Upload the following code to an Arduino board

   void setup() {
     Serial.begin(9600);
   }
   void loop() {
     byte datapoint = 0;
     while (datapoint++ < 9) {
       Serial.print(random(100));
       Serial.print('\t');
     }
     Serial.println();
   }

2. Select Tools > Serial Plotter from the Arduino IDE menus.

Note that the Serial Plotter window is blank:

Requested behavior

Communicate to the user when the limits of the plotter have been exceeded.

Desktop

• OS: Windows 10
• Version: 2.0.0-rc1-snapshot.f0d9894
  Date: 2021-11-30T16:25:41.414Z
  CLI Version: 0.20.1 [abb21449]

Additional context

Since the classic Arduino IDE's Serial Plotter does not have this limitation, even experienced users users will not be familiar with it.

[per1234]

Fixed by arduino/arduino-serial-plotter-webapp#5
Even though that PR has not yet been merged, the fix there was already published to the npm registry and brought into Arduino IDE by #698