Fix number of horizontal ticks when defined by callback function.

[markruys]

When you want to specify which ticks you want to show up on the horizontal axis, you typically use a callback function in options.xAxes[].ticks. As shown in https://jsfiddle.net/xuf1qwtp/, this goes wrong for larger sets of values: only a single label gets drawn. The pull requests fixes this.

[etimberg]

I think this is a step in the right direction. One case that might not be handled well is if the callback does not hide labels at regular intervals. I was recently looking at overhauling the auto skip code to work for vertical axes and in doing so I thought about writing the auto skip to actually check if the text was going to overlap and only hiding the label if it would.

@simonbrunel @zachpanz88 what are your thoughts on merging this as a stop-gap? I almost think we should iterate a bit per my notes above.

[markruys]

I think the fix does work well for the case the labels are spread irregularly. This is because skipRatio will be set to false (assuming you do have enough space to position the labels), hence the shouldSkip won't get set.

I confirmed code like in https://jsfiddle.net/xuf1qwtp/ with

var options = {
  scales: {
    xAxes: [{
      ticks: {
        callback: function(value, i) {
          return Math.random() < number_of_items_on_xaxis / number_of_items ? value : null;
        },
      }
    }],
  },

and it works.

[etimberg]

Closing as out of date