[BUG] Use time.unit option to create default min/max for empty chart

[jcopperfield]

Currently the default min/max values for the time scale are create with the day unit.
If the user has set the time.unit option and has hidden all the datasets, the scale will be empty (no tick labels).
Using the time.unit option will show the labels as selected.

[simonbrunel]

Looks good, I would add unit tests next to this one and this one that check usage of time.unit when explicitly set.

[etimberg]

I agree with @simonbrunel about a test. Other than that, looks great!

[jcopperfield]

@simonbrunel I don't know what you are expecting the code to do in those 2 test situations. The PR sets the scale limits for auto generated timestamps. When ticks.source is labels or data the result would still be an empty, due to the code in buildTicks.

[simonbrunel]

It's not specific to auto generated timestamps but to all cases where there is no labels or data or explicit min/max to compute a valid data range. If ticks.source == 'labels' | 'data' and if there is no labels or data, the previous behavior was to generate a min/max range for a whole day. Your PR changes that behavior when there is an explicit time.unit so we should have this kind of tests:

it ('should correctly handle empty `data.labels` using `time.unit`', function() {
    var chart = this.chart;
    var scale = chart.scales.x;
    var options = chart.options.scales.xAxes[0];

    options.time.unit = 'year';
    chart.data.labels = [];
    chart.update();

    expect(scale.min).toEqual(+moment().startOf('year'));
    expect(scale.max).toEqual(+moment().endOf('year') + 1);
    expect(getTicksLabels(scale)).toEqual([]);
});

We should also rename tests I linked for something more explicit, like:

it ('should correctly handle empty `data.labels` using "day" if `time.unit` is undefined', ...

[simonbrunel]

Thanks @jcopperfield