I was recently asked to explain chromaticism, so I thought I'd give it a try here.
Bach's fugue based on a theme by Tomaso Albinoni in B minor, BWVs 951, and 951a: the subject itself contains a chromatic descent (B, A#, A, G#, G, F#) and though Albinoni himself wrote it, it was Bach who brought the ideas behind such chromaticism to the next level. 
HERE is the recording which I was not able to credit. If you know whose recording this is, please write me, I'll fix it!
and HERE is the version (BWV 951, not BWV951a) which I played on the MIDI harpsichord, highlighting the chromatic lines on one of my favorite youtube channels. 
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