[BUG] Can't update scale.ticks.* options at runtime since 2.7.0

[VerifiedJustLooking]

If you look at this jsfiddle: http://jsfiddle.net/jmpxgufu/179/

I put together a small bar chart, and on an interval (after 5 seconds), it will update some fontSizes and labels on the y-axis. Yet, the tick fontSize won't update (but everything else will).

I am pointing to https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.0/Chart.bundle.min.js (or if you try to point here: http://www.chartjs.org/dist/2.7.0/Chart.bundle.js, it fails as well, it doesn't matter either way -- here's a jsfiddle using that lib: http://jsfiddle.net/jmpxgufu/183/)

I had stolen this jsfiddle from someone and modified it to fit my needs. Previously they were using 2.4.0, and it worked with that library. Did something break between 2.4.0 and 2.7.0?

[jcopperfield]

It seems that v.2.7.0 uses the ticks.minor and ticks.major options to set the font.
Using myLiveChart.scales['myAxis'].options.ticks.minor.fontSize = 36; in the example will set the fontsize correctly.

[jbgtmartin]

It worked for me thanks a lot! I had the same issue with ticks.fontColor and ticks.display.

The docs tell that if they are not specified ,ticks.minor and ticks.major properties inherit from ticks. It is the case when the chart is initialized, but I suppose that when calling chart.update() it doesn't work anymore because ticks.minor and ticks.major are already filled with the initial values so they don't inherit the new ones and we have to edit them manually.

[jcopperfield]

@simonbrunel I might be mistaken, however it seems that PR #4198, actually fixed this bug by recreating the scales on update.

[simonbrunel]

That would need to be tested but I don't think #4198 fixes that issue: scale options are not generated but fetched from the chart.options (eq. config.options). When the chart is created, the scale class generates the minor and major ticks options from ticks under scale.options.ticks.minor/major. At the next update, chart.options.xAxes[index].ticks === scale.options.ticks and already contains minor/major.

Ideally, scale.options should be renamed to scale._options (private) and should always receive a copy of the user options. That would make easier transforming scale._options in each scale without worrying about breaking the user initial options (which IMO, should never be modified by chart's internals).

Though, I think that would be a breaking change since scale.options is public.

[simonbrunel]

Not tested but what we could do in core/core.scale.js:

Chart.Scale = Element.extend({
  initialize: function() {
     // initialize is called from the Element constructor
     this._update();   
  },

  /**
   * Called by the chart controller on update()
   * @private
   */
  _update: function(options) {
     // Note: keep this.options valid for backward compatibility
    this.options = options;
    this._config = this._configure(options);
  },

  /**
   * @private
   */
  _configure: function(options) {
    // Let's first clone `options` which is probably a reference 
    // on the user options and should not be modified internally
    // https://github.com/chartjs/Chart.js/issues/4896
    var config = helpers.clone(options);
    var ticks = config.ticks;
    
    // ... resolve config.ticks.minor / config.ticks.major ...

    return config;
  }
})

Then replace all uses of scale.options by scale._config.

And in core/core.controller.js:

scale = scales[id];
scale.ctx = me.ctx;   //< not sure why it's needed 
scale.chart = me;     //< not sure why it's needed
scale._update(item.options)
// ...