[FEATURE] add option to order axes

[cizmiak]

Expected Behavior

If i set order number property in scale options for some axes, these axes should by ordered by this number.

Current Behavior

There is no such option. Now, axes have same order as results from Object.keys(chart.scales) which is arbitrary and could be unexpected.

Context

Enable to user to set axis ordering.

Environment

• Chart.js version: 2.5

[etimberg]

@cizmiak you're in luck! We recently added some code to allow arbitrary position of other items on the canvas (legend, title, etc) to ensure they can be sorted by a weight field. We made the underlying code generic enough so it should be easily adapted to axes too!!

When items are added to the layout system via addBox we check to see if layoutItem.weight is defined and default it to 0. https://github.com/chartjs/Chart.js/blob/master/src/core/core.layoutService.js#L65-L67

The axes get added to the layout system here: https://github.com/chartjs/Chart.js/blob/master/src/core/core.scaleService.js#L39

When the axis is constructed here you could set some weight if it's set in the options. In the legend the value of the weight parameter is hard coded.

I'm happy to look at a PR for this :)

[cizmiak]

@etimberg , thanks for giving me direction.

[etimberg]

Done in #4094