Non-scaling SVG scatter points

[rreusser]

See issue: #760
See related line scaling PR: #761
See codepen example (visible on scrollZoom): http://codepen.io/rsreusser/pen/OXvmjR?editors=0010

This PR compensates for point scaling by applying an inverse scale transform. It's simple, but kind of an ugly hack since it requires stripping any existing scale from points before applying a new scale. That may make it prohibitive, but it was a bit easier than expected, so maybe it's not unreasonable. cc @mdtusz

[rreusser]

This is scatter-specific. It's here since related functions are right next to it, but it probably should not in lib/. I just don't think there's a world in which this can be meaningfully generalized.

[etpinard]

I'm ok with this being in lib/. Putting this in src/traces/scatter/set_point_scale.js would be ok too. Your call @rreusser

[etpinard]

a one-line comment here would be nice.

[etpinard]

Nice solution. Looks like isolating the setPointScale call in dragbox.js worked out well.

@rreusser Would you mind adding a few unit tests for setPointScale ?

[rreusser]

@etpinard Should setPointScale work with d3 elements, like setTranslate? As long as it's clearly implemented, tested, and commented, it seems unnecessary to implement at the moment unless needed.

(Specifically, there's a danger of it being a bit expensive when lots of zoom events are happening since it applies to all points, so it's not totally general and abstract)

[etpinard]

Should setPointScale work with d3 elements, like setTranslate?

No. I don't see the need.

[rreusser]

Added comment, changed the name to be more descriptive (setPointGroupScale since it operates on a selection—although that name doesn't seem quite right either since it's not an svg <group>), added tests, removed scale if = (1, 1), and confirmed that it still has the same visual effect.

[etpinard]

Very nicely done. 💃 merge away.