strokes fork and port of mike's general update pattern tutorial, I.
merge requests should go to the example in the repo
This example demonstrates the general update pattern in D3, where a data-join is followed by operations on the enter, update and exit selections. Entering elements are shown in green, while updating elements are shown in black. Exiting elements are removed immediately, so they're invisible.
This example does not use a key function for the data-join, so entering elements are always added to the end: when the new data has more letters than the old data, new elements are entered to display the new letters. Likewise, exiting letters are always removed from the end when the new data has fewer letters than the old data.