GeoSonix is a free open source graphical music sequencer. It's now available for Mac and Linux and Windows.
The latest beta version (free and open source) is available for download from bitbucket.org/chrisgr99/geosonix/downloads Be sure to always choose to download the most recent version from the bitbucket site.
This example uses a single cursor travelling on a path over an image to create a piano composition. The synth is the Ableton Live Acoustic piano.
As the cursor travels, the midi note-on message rate varies between 30 and 500 milliseconds in proportion to the blue channel of the pixel behind the cursor.
The note pitch is determined by mapping the red channel the pixel behind the cursor into the current score harmony which is defined as 3.2 octaves of melodic minor scale starting at the note A1. Note velocity is controlled by mapping the green channel of the pixel behind the cursor into a velocity range of 60 to 127. At the same time midi cc 1 is being generated based on the cursor X position, and is controlling the "motion" parameter of the piano patch.