I suggest making the landing page a bit more clear... I sat there looking at the single sentence thinking, "What?" It's not so much a missing link between anything, as it is a way to create visualizations from your spreadsheet data, if you feel comfortable enough to paste it on a random website.
"Create visualizations from your spreadsheet data"
That's what this does. The "missing link" cliché didn't score any points with me, but the product looks extremely cool.
One other point of feedback, I clicked the "How it works" link, and when the page scrolled down, the animation was already on step 4, but that text is small, so I didn't realize I was watching the last step. I was a bit confused.
Maybe consider using a scroll position detection library to hold the animation until the user has it in view, or use something more clear to indicate which step is being illustrated:
For me the missing link would be a clean way of getting Excel data into a format that could be read by my D3 scripts without needing custom macros, exporting to Excel's flaky CSV format and a manual reload at the end.
One good reason would be (as he says) that it is flaky. eg how does it handle newlines, commas, tabs and non-ascii characters in data? I've had to pull dirty data between systems before, and csv as the lowest common denominator tends to be lossy as there's no spec on this stuff. Some systems (eg oracle's csv import) do support forms of escaping, but they're not interoperable.
I mean that I want to use Excel as a spreadsheet and use its contents to drive a D3 diagram without the current hassle of getting the data out of Excel into a format that D3 can read cleanly (and continuously - if I update the spreadsheet, the diagram should also update).
