[color-chart] How should a chart look if the underlying `<color-scale>` has only one color?

For now, it throws because [the `getAxis()` function](https://github.com/color-js/elements/blob/5cf02760b3a971173d29cc7989c74b41548c167d/src/color-chart/color-chart.js#L218-L241) returns `NaN` for `min`, `max`, and `steps` when calculating the corresponding props for the X-axis.

I checked what Excel does when it gets only one data point. It looks like Excel calculates the range based on an ”assumption“ that the chart should look nice. If I understand correctly, it uses 1 as the X coord (if not explicitly specified). I suggest that the one data point will never be outside the range.


We can probably do something similar (for the range of X).