This is quite useful 

Try the JS MIDI Chord In Key[midi/midi_chordkey]plugin. It's a good start

I can't tell if you're on Windows or Mac. If Mac...never mind. If Windows, there's a cool, inexpensive app (US$29) called ChordPulse. Really easy to use. You compose in CP, then drag the MIDI to wherever. Perfect timing. New YT video from Kevin Wayne...using ChordPulse with Reaper. A longer (5 x 15 minutes) YT video series on a previous version (2013).

Mochordz 

pizmidi "Account Suspended" 

"There's also PizMidi midiChords Went there today... "ACCOUNT SUSPENDED"

OP is likely long gone but for future enquiries, I think Scaler is exactly what he's looking for. You can even display as a guitar fretboard instead of keys. Great functionality.

Oddly enough, I just turned 50 and am revisiting 15 because I've just got a cheapo Strat type to mess around with. Mid-life crisis possibly, but I just fancied it. And I can damn well afford a reasonable knockoff, so why not? (Incidentally for those who don't know, when you're my age you buy presents for yourself...) Back in the day, piano was always my instrument but when I didn't have one available I'd play a guitar. I could do okay, but I was never a god at it. Having since lived through some 30-odd years as a pianist, I find it interesting that my mental and physical approach and attitude is still so different on a guitar. I've played with and written for guitarists, but overwhelmingly just given them leadsheets and let them sort it out. No criticism intended either way, just sayin' (as they just say).