Overhaul zoomable behavior

[jrideout]

Fixes #253

See the spec

Note, this depends on #399

[jrideout]

Note: this is not ready yet. Adding the PR to track progress

[matthull]

@jrideout I'll be a few more days on this one; limited time at the moment, and want to add a pretty comprehensive test suite for coordinate chart as a safety net since the zoom changes are so impacting. When I'm done I'll rebase against master and account for #403 as well if that isn't merged yet.

[matthull]

@jrideout this is about ready to go and rebased last night - however since there are lots of changes to coordinate-grid-chart in other pull requests, I'll wait until this weekend when things are quieter to do final rebase. Should be ready for review next Monday.

[jrideout]

@matthull yeah - the pivotal folks have been prolific. This PR touches enough of codebase, that I'd like to get it integrated so you don't have to keep tracking master and so that others can build off it. I think we can have a "Merge Freeze" over the weekend to support your efforts.

@NickQiZhu @gordonwoodhull, if you guys agree, let's hold-off on merges until we get this one merged on monday.

[matthull]

If you want me to go ahead and get this one ready to go tomorrow ahead of #415 I can do that too - just let me know. Mainly trying to avoid doing several more rebases on this PR as new stuff comes in, and since #415 is still in progress I didn't know when would be the right time to pull the trigger.

[jrideout]

@matthull - go ahead. We can wait on 415. Fixing the existing featureset has higher priority than new features.

[NickQiZhu]

Sounds good, I only have enough bandwidth recently to answer questions on user group anyway =)

[matthull]

This is ready to go. All the issues I listed in the wiki article originally are fixed. Two issues I added to the article later (box plot/ordinal and line hover issues) are not fixed. After this is merged I can update the article to indicate which issues are fixed, and perhaps open a ticket for the line hover issue.