fix entering transition positions

[gordonwoodhull]

It seems that the default value must have changed for attributes which were not specified in the enter step but then transition in the update step.

Whereas before the unspecified initial position seemed to just take the final position, now it seems to start at zero.

I did not find any code changes in #1363 to explain this, so I reluctantly blame d3v4. Or I don't know, maybe it's a better, more consistent behavior, but it breaks all the things listed in #1363:

bar charts

• introductory transition on bars is from lower left instead of just from bottom
• bar labels animate from upper left instead of just being there at start

line charts

• dots animate from upper left instead of just being there

heat map

• squares animate from upper right instead of just fading in
• y axis ticks animate from top instead of just being there

and there are likely to be more. Seems like it would affect colors as well.

It's probably good to fully specify initial values on enter, even if it's a little tedious sometimes

[gordonwoodhull]

Partial explanation of what changed: https://stackoverflow.com/questions/47339762/what-attributes-does-d3-transition-change

[gordonwoodhull]

I think I got all of them. 3.0 alpha 6