A few years ago I saw a talk that described a tracing tool for blocks-based programming languages. If I recall correctly, the idea was to show time on the horizontal axis, and to display “ghosted” images of blocks with variables’ values, graphical snippets to show drawing and motion, and so on. It looked something like this:

Tracing Execution of Blocks

except it had a grid background and a slider control to allow the user to scroll backward and forward in time.

I honestly can’t remember where I saw this, who presented it, or whether it was a proposal or an actual tool. I’d be grateful to anyone who could point me in the right direction.