[BUG] Chart.js grid lines and axes not working in v2.7.1 as they did in v2.4.0

[seancclark]

Appear to have found a bug (or something has changed between version 2.4.0 and v2.7.1).

Working example with v2.4.0: https://jsfiddle.net/juk94xpo/
However chart doesn't look as it should if you upgrade to v2.7.1: https://jsfiddle.net/1sqkztto/

It seems that v2.7.1 does not draw tick marks in the correct position and axes disappear. This worked correctly in v2.4.0.

[etimberg]

Thanks for reporting this @seancclark. I looked at both fiddles and the canvas is the same size in both.

The issue with the grid lines looks like it changed in v2.5.0. I think the change boils down to starting with v2.5, the step is incremented from the axis minimum value while in older versions it seems to have been centered around 0.

I think a different, easier to use, mechanism needs to be created for specifying tick marks at certain values.

@simonbrunel @benmccann how would you feel some of the following options:

• An optional setting that is an array of tick values, and if specified, overrides min, max and stepSize
• Allowing a function to be specified for the ticks (using the scriptable options perhaps?)

[simonbrunel]

An optional setting that is an array of tick values, and if specified, overrides min, max and stepSize

Isn't it the same as the scale labels options (#4506), but applied to all scales?

Allowing a function to be specified for the ticks

What would be inputs/outputs of this function? Scriptable options are mapped to the dataset/data, doesn't really make sense for ticks.

[etimberg]

Yup, like labels but in the scale options.

options {
  scales: {
    yAxes: [{
      ticks: {
        values: [0, 19, 43, 50] // implies min == 0, max == 50
      }
    }]
  }
}

The function idea would take the data range and return the ticks. Ticks outside the range would be ignored.

options {
  scales: {
    yAxes: [{
      ticks: {
        generator: function(range)  {
          return [0, 10, 20];
        }
      }
    }]
  }
}

[simonbrunel]

#4506 is about labels in the scale options, but only for the category scale IIRC, so would generalize this concept to all scale for consistency.

[etimberg]

Yup! That would fix 2 issues at once 😄

[jcopperfield]

@etimberg I think having different configuration options tick.values and tick.generator is overkill. I think the ChartJS code should be smart enough to check the type of the option.
e.g.

if (typeof values === 'function') {
    values = values(generationOptions, dataRange);
}
if (!values) {
    // generate default
}

[simonbrunel]

Agree with @jcopperfield, that's what we do with scriptable options. Though, I would keep the scale.labels options introduced in #4506 (for consistency) and generalize this feature to all scales (should be relatively easy at the core.scale.js level).

options {
  scales: {
    yAxes: [{
      labels: [0,10,20],
      // or
      labels: function(context) {
         // context: {chart, range?, ...)
         return [...];
      }
    }]
  }
}

Actually, you might be right @etimberg, that could be a scriptable option with a scale specific context.

[jcopperfield]

@simonbrunel didn't know there scriptable options were already implemented. However wouldn't it be better to call the scriptable option function using value.apply(context, arguments_array);?

[simonbrunel]

Why would it be better?

[jcopperfield]

So that someone trying to override the function would get the same arguments as the original function. For instance for the generator function(generationOptions, dataRange) and for a formatter function(tickValue, index, ticks) instead of having a context object.

[simonbrunel]

That would make the whole scriptable concept hard to understand and maintain because not consistent. I doubt there is any advantage to map the generators/formatters signatures with our options. The context object should be simple and the same for all options under scale, making also easier to share these functions between options: the context is not related to the option itself. Also keeping the function signature simple (only a single argument) helps to prevent breaking changes when introducing/deprecating properties since we don't care about the arguments order.

In the case you want to write a custom generator/formatter, then it's easier to override/augment these methods directly (there is a ticket to make them public). If you want a very custom labels array, based on the chart data for example, then create a scriptable option and do whatever you want with context.chart.data or context.scale for example. You have almost access to everything and I don't think we should expose more specific properties to this context (I would not expose any range).