Use maximum step as default for single step selection.

[bmd3k]

Motivation for features / changes

We are changing single step selection to default to the maximum step. We think this is more useful than making the default the minimum step.

This also means that when step selection changes from single selection to range selection, the minimum value of the chart is added as the start of the range and the previous single step now becomes the end of the range. Similarly, when step selection changes from range selection to single selection, the new single selection is the previous maximum value of the range.

Technical description of changes

Where default value is set to "min" step, instead set it to the "max" step. Where range selection is enabled, ensure the new start of the range is the "min" step.

I have three internal CLs that accompagny this change.

• cl/528745120 adjusts or comments out code to allow the import to succeed. It will be submitted prior to merging this PR.
• cl/528745440 is a set of harmless screenshot-updates. It will be patched into the import CL and submitted at the same time as the import.
• cl/528745657 is a set of changes to uncomment out the code from the first CL and make remaining adjustments. It will be submitted after the import.

Detailed steps to verify changes work correctly (as executed by you)

• I tested it manually quite a bit.
• I imported it into the internal repo and ensured that end-to-end tests pass (after adjusting for the change in behavior).

[rileyajones]

This is a bit confusing at first. But with the context I have around the problem, this makes sense as a solution.

[rileyajones]

As discussed in #6364 I would love to remove the ability for end to be null. If range selection is disabled then the nd value shouldn't be used so I don't see a reason to remove it here.

Suggested change

	timeSelection = {
	start: timeSelection.end ?? timeSelection.start,
	end: null,
	};
	timeSelection.start = timeSelection.end ?? timeSelection.start;

[bmd3k]

I don't intend to change the data model as part of this PR. Besides, I don't really agree with the sentiment that motivates that particular change.

I set end to null here to be clear that we are returning to single step selection. The end gets set to null, anyway, as a side-effect of calling formatTimeSelection but I find it unlikely that somebody will read this code and understand that.