I offer over 30 years hardware/firmware development experience. Since the late 1980's I have developed commercial products based on Motorola 6800/68000 families, Intel 386EX, Hitachi/Renesas H8/300, Microchip PIC, both in assembler and ANSI C.
You request 68HC11 as a target. I have some questionas/considerations. This is a very old (over 20 years) device only made by second sources in Asia. As such for a new design one should use more current microcontrollers. Of course if you have a demo/development board and only plan to program one unit, not for commercial production, then any current or older microcontroller is fine. However for a new design I would strongly recommend a current, latest technology microcontroller as that would be well supported into the next several years.
You also suggest C source programming. That is fine for current microcontrollers, but for such old devices as 6800 the possabilities will be very limited. No one supports development environments for that any more. Those that remain from back then were written to run on MS-DOS machines (286, 382, etc.). They will still execute in a command window under Win XP. So for 68HC11 you will be limited as to whatever old dev tools are still around.
Either way I can provide what you need. You also state you desire an Excel compatable output. That would be a .csv file format. You just receive the data to a file, close the file after all desired data is received, then open it with Excel to prepare the plot.
Realize the only products that have such graphing and display capability built in are based on a high end microcontroller, likely running an OS, and with a full color LCD screen. These are typically instrumentation systems in the 1000's of dollars. I do not believe you are looking anything like that.
I am in US, south Texas.