use xUnits for axis padding

[gordonwoodhull]

@mtraynham noted in #892 (comment)

So the coordinate-grid chart currently has xUnits which could potentially be reused for this purpose. xUnits supports all the d3.time[t].range functions (i.e. d3.time.minutes, d3.time.hours), which are aliases to the range functions of intervals (i.e. d3.time.minute.range, d3.time.hour.range).

Maybe instead, we have xUnits just take the top level interval functions (i.e. d3.time.minute, d3.time.hour). It's typically the same thing that is passed in for scales. For brushing, we'd call the range function where appropriate and for padding we would call the offset function.

To support that, we'd have to update the dc xUnits variants for integer/float/ordinal. Maybe something like:

dc.units.integers = {
    range: function (start, end, step) {
        if (!step) {
            step = 1;
        }
        return Math.abs((end - start) / step);
    },
    offset: function (num, step) {
        return num + step;
    }
};

dc.units.ordinal = function (domain) {
    return {
        range: function (start, end, step) {
            return domain;
        },
        offset: function (value, step) {
            return value;
        }
    };
};

dc.units.float = function (precision) {
    return {
        range: function (start, end, step) {
            if (!step) {
                step = 1;
            }
            var d = Math.abs((end - start) / (precision * step));
            if (dc.utils.isNegligible(d - Math.floor(d))) {
                return Math.floor(d);
            } else {
                return Math.ceil(d);
            }
        },
        offset: function (num, step) {
            return num + precision * step;
        }
    };
};

We then could reuse all these functions for padding, although, we may need to support yUnits as well. As you said, this doesn't address the percentage padding problem, but these solutions don't necessarily have to be done together.

[gordonwoodhull]

I don't know why I didn't see this before: it's trivial just to make dc.utils.add / dc.utils.subtract / coordinateGridMixin.xAxisPaddingUnits take the d3 interval object instead of the name of it.

There was no need to change xUnits since this is already what xAxisPaddingUnits does.

Fixed in 3.0 alpha 11. Commit (with dyslexic reference to this issue) 5776594