I have discovered that many XON / XOFF terminal programs require a delay between characters when sending gcode data to my plotters. 
While a delay ensures reliable communication, any delay slows the rate at which the gcode commands are sent which lengthens the time taken to plot.
My terminal program solves this problem. 100% reliable data communication is now achieved without the need for any delays. Files that took hours to plot now take minutes :)
 A list of my plotters follows:
Step 1: Software
This gcode terminal requires that you have "Processing 3" installed on your computer.
"Processing 3" may be obtained from https://processing.org/download/?processing
Step 2: Installing the Gcode Terminal Software
"Processing 3" is very similar to the Arduino IDE (integrated development environment).
Run "Processing 3" and paste the contents of "Processing3_terminal.pde" into a new sketch.
That's it !
Step 3: Testing
The "CNC Drum Plotter" will accept either Bluetooth or USB output from the terminal.
If you choose to use Bluetooth you must enable Bluetooth on your computer and "pair" your computer with the plotter. If you choose to use a USB cable then uplug the HC-06 Bluetooth module from your plotter.
Copy the attached file "Letter_B.ngc" into your "Processing 3" folder. This file tests each of the G00, G01, G02, and G03 functions. The end result should be a large A4 size portrait image of a capital letter 'B'.
With your "Processing 3" sketch running drag the horizontal bar upwards to maximize the visible terminal area.
Now click the "Run" button which is on the top-left corner of the screen and follow the on screen prompts.
If the terminal doesn't connect change "DEBUG_ON = false;" to "DEBUG_ON = true;" and match the terminal's "COM" port setting to that used by the plotter.
Once you have positioned the plotter's pen at the bottom-left corner of your A4 page, "right-click" the instruction box and type "Letter_B.ngc" (without the quotes). Keep in mind that the terminal is "case-sensitive".
All going well gcode commands, echoed back from the plotter, should start streaming down your screen and the plotter should start moving.