[SamBam]

I've been using D3 for charts for a while. I'm not a power-user by any means, but I understand it enough to make basic plots.

When would I use this instead of straight D3?

[mbostock]

Most of the time. Unless you want or need a bespoke visualization, Plot will let you construct a meaningful plot with much less time and effort. (That said, Plot is brand new and still under development, so it might not yet have the specific features you need.)

[vermarish]

When you only care about which visual encodings best represent your data, and not how those encodings are implemented in SVG/CSS.