Mod 1: I bought this $7.50 12V -> 9V DC converter from Amazon and am using its 9V to feed the printer. I can report it does print faster. I can probably tweak other delays out of the system??
Mod 2: I removed the SPCR=0; from the printGraph function where my setup previously lost its mind.
Observation: My actual problem with the program "losing its mind" was my misuse of SPCR. Coincidentally, 5V really isn't enough to feed that printer. What sucks is I built my design around that nifty 5V/12V power supply. Ah well.
Current status: I can print the sample empty box in its entirety from my function. The print stalls in the middle and pauses for quite some time. The thermal.h code has a lot of delay added in which probably can be removed now that I'm using proper 9V power. So now I can go back to trying to read in the sample empty box, altering the few bits I need, then writing it to the SD Card to then be read and fed to the printer.
I just discovered chipKIT and its uC32. With 32K of flash, I can handily process a 19K array all within SRAM and be rid of micky-mousing around with the SD Card. It shows up tomorrow... we'll see if all this stuff is really "easy" when I transfer this reality over to there!
1) Try the uC32 and see if it works. I also ordered the proto board so I can attempt to rebuild my whole setup on the ChipKit stack.
2) If that doesn't work, I'm going to try and make SPI RAM work for my Mega. I'm really really over my situation of being size limited when trying to output the graph to the thermal paper. I would already be done with this project if the Mega had 32K of SRAM. ANY manufacturer making their own Mega really should find a way to get the "external" SRAM on-board, even if unpopulated. My Seeedstudio Mega has plenty of room for it on the PCB.
I have looked at Raspberry PI, the PC Duino, and the Beagle thing. I really don't want to have my gizmo running linux and to have to boot up. My first real SE job had vxWorks running on our equipment and I swore off ever using anything OS-like for embedded.