I've looked at the help files and been all over google, but I can't find a way to add grid lines and scales to an array.
How could I do something like in the attached picture using PD - keeping in mind that I need to change the grid markings and intervals depending on the data to be displayed and that I would really like to be able to change the size of the array display while the program is running.

Is there anything built in to PD, or is there and external, or will I have to bodge something together to get the effects I want?

Check the array help, in a sub-sub-sub-patch : all about arrays/how to manipulate arrays/additional tools you'll find what you need.

OK. Found it. If you go through the PD help browser, then under 5.reference you can find all_about_arrays.pd
There is then a patch named How_to_manipulate_arrays_in_PD in that patch is another named Additional_tools which finally shows how to set ticks and labels.
While I was digging, I found another patch that shows how to get selected values - which I also need.
There seems to be a way to change the size of the array, too.
I'll see if I can mix it all together and get what I need.

Those help patches don't have everything you need because Pd's graphs only have "tick" marks. There isn't an easy way to make grid lines.